WILKINS, J.

We reverse the defendants' convictions because, in violation of their constitutional rights, the trial judge barred the defendants from a portion of their trial during which testimony was presented. The judge excluded the defendants, but not their counsel, from the courtroom during the testimony of a police officer concerning the location from which, as he testified, he saw the defendants engage in unlawful sales of drugs. The judge ruled as she did in response to the prosecutor's request that the place from which police officers saw the illegal activity not be disclosed. The judge instructed counsel for the defendants not to identify the witness's observation point to their clients.

The exclusion of the defendants from a portion of their trial violated their constitutional right to confront witnesses against them expressed in art. 12 of the Massachusetts Declaration of Rights. The State's interest in the confidentiality of a surveillance point may not be permitted to override a defendant's constitutional right of confrontation. There may be circumstances in which the Commonwealth is privileged not to disclose a point of surveillance during trial without violating a defendant's rights to confrontation and to a fair trial. The confidentiality of a surveillance site may not, however, be preserved by permitting the trier of fact to hear testimony from a witness outside of a defendants' presence.

The defendants were tried along with four other persons in a jury-waived trial at which the others were found not guilty. The defendants before us were convicted of trafficking in cocaine and of conspiracy to traffic in cocaine. The defendant Rios was also convicted on two counts of unlawful distribution of cocaine. We transferred the defendants' appeals here on our own motion.

The Commonwealth's case was based on a purchase of cocaine by an "undercover" police officer, the testimony of Springfield police Officer John Leonard who had observed activities in front of a house on Medford Street in Springfield for several hours on the same day as the "undercover" purchase, and evidence seized pursuant to a search warrant issued and executed the day after the "undercover" purchase.

Officer Leonard testified that on September 22, 1988, he conducted a surveillance of a residence at 14 Medford Street "[f]rom a secure location with a direct view of the home" at a distance of approximately twenty-five yards "[i]n the general area" across the street. The officer described various transactions that could have been found to have been transfers of plastic bags in exchange for money. Immediately in cross-examination, counsel for one of the defendants inquired of the officer as to exactly where he was. There was an objection by the prosecutor, followed by an extensive discussion between the judge and counsel. The prosecutor asserted that the site of the surveillance should be treated as privileged and cited Commonwealth v. Lugo, 23 Mass. App. Ct. 494 (1987). The Commonwealth argued that Medford Street is in a high crime area and that the police would lose the use of the vantage point if the site were disclosed. The judge conducted a hearing on the question whether the surveillance site should be kept confidential. There was evidence that a person could see the full length of Medford Street and a portion of Main Street from the site, which had been used many times before, and that, if one knew where to look, a person conducting a surveillance there could be seen.

The judge found that, if the location were to be disclosed publicly, the police would lose the site as a useful place for surveillance. She made no finding whether the disclosure of the site would place police officers in danger. She ruled preliminarily that the specific location might be entitled to protection from public disclosure. She then conducted a further hearing, in the absence of the defendants, concerning the exact location and whether it should be disclosed. Counsel were ordered not to reveal any information received during the hearing. There is no doubt that counsel objected to the absence of the defendants on confrontation and effective assistance of counsel grounds. Officer Leonard testified at length about the surveillance location and what he could see from it on September 22, 1988. The judge took a view.

In the absence of the defendants, the judge stated what she had seen during her view of the surveillance site, heard argument from counsel as to whether the location should be disclosed publicly, and found that the view from the surveillance site was unobstructed. She ruled that the government had a substantial interest in keeping the location confidential and that the defendants would not be prejudiced by not being present during Officer Leonard's testimony concerning the surveillance site because their counsel would be present, because counsel had heard the judge's observations concerning her view of the site, and because the subject of the testimony was physical observations. The judge ruled, however, that defense counsel would be entitled to ask Officer Leonard, in front of the defendants, whether he was in a motor vehicle.

The voir dire hearing ended and the judge heard cross-examination and redirect examination of Officer Leonard, in the absence of the defendants, concerning his surveillance point and what he did there. In fact the questioning went well beyond the subject that the judge ruled should be kept confidential. All the in camera testimony bore directly on the defendants' guilt. When questioning about the surveillance site had concluded, the defendants were readmitted to the courtroom, and cross-examination of Officer Leonard continued. During that testimony, the officer testified that he had been in a motor vehicle and that anyone on the porch of the Medford Street house who had looked in the right direction could have seen where he was twenty-five yards away.

There is no doubt that generally a defendant has the right under art. 12 of the Declaration of Rights to be present during his trial. Commonwealth v. Bergstrom, 402 Mass. 534, 543 (1988). We are aware of no case that holds that it is constitutionally permissible to bar a defendant from seeing and hearing the trial testimony of a witness against him, except in those situations in which the defendant's voluntary conduct has led to that result. See Commonwealth v. Chubbuck, 384 Mass. 746, 751 (1981) (persistent disruptive behavior in the courtroom); Commonwealth v. Senati, 3 Mass. App. Ct. 304, 307 (1975) (same); Commonwealth v. Flemmi, 360 Mass. 693, 694 (1971) (voluntary absence from trial constitutes a waiver of right to be present). Article 12 includes the right of a defendant to have the witnesses confront the accused face to face. Commonwealth v. Gallo, 275 Mass. 320, 333 (1931). See Commonwealth v. Bergstrom, supra at 544 ("we have never interpreted art. 12 as permitting introduction of an available witness's testimony outside a defendant's presence"). In the Bergstrom case, although the defendant saw and heard witnesses' testimony through closed circuit television transmission and could communicate with his attorney during that testimony, this court held nevertheless that the defendant's art. 12 right of confrontation was violated. Here, the defendants were denied the opportunity to hear a portion of the testimony of an adverse witness and to communicate with counsel concerning that testimony.

The denial of the defendants' constitutional right to be present during the cross-examination and redirect examination of Officer Leonard cannot be justified by the Commonwealth's interest in the confidentiality of the surveillance site. See Commonwealth v. Stockhammer, 409 Mass. 867, 883-884 (1991). The only justification for the claimed confidentiality is that the police would like to use the location again. It might be that that sole reason for asserting the surveillance location privilege would justify preserving the confidentiality of a site. The result of a decision to preserve confidentiality should not, however, be the removal of a defendant from the courtroom, but rather a ruling that the location not be disclosed during the trial. It was the justification for confidentiality that the Appeals Court and then this court considered in Commonwealth v. Lugo, 23 Mass. App. Ct. 494 (1987), S.C., 406 Mass. 565 (1990). There is nothing in those opinions supporting the involuntary exclusion of the defendants from their trial.

The Commonwealth does not strenuously argue that the removal of the defendants from the courtroom was not error. Rather it contends that any error does not require reversal of the convictions. It defends the judge's determination that the surveillance location should be kept confidential and argues that there was ample evidence to support the defendants' convictions quite apart from the testimony of Officer Leonard.

Even if we were to accept the argument that the violation of a defendant's art. 12 right of confrontation could be harmless error, the Commonwealth must make an affirmative showing that the error was harmless in order to overcome the presumptive prejudice of the constitutional violation ( Commonwealth v. MacDonald (No. 1), 368 Mass. 395, 399 [1975]), and must make that showing beyond a reasonable doubt. See Commonwealth v. Hanger, 377 Mass. 503, 510 (1979). It is, of course, impossible for the Commonwealth to demonstrate that, if the defendants had been in the courtroom, the effect of Officer Leonard's testimony on the trier of fact would have been no less beneficial to the Commonwealth than it was in their absence. We cannot know, for example, what the defendants might have beneficially contributed to the cross-examination or rebuttal of the police officer. See Commonwealth v. Robichaud, 358 Mass. 300, 303 (1970). Nor can we know how his testimony, which was both incriminating and corroborative of other testimony (but not merely cumulative), may have affected the finding of guilt beyond a reasonable doubt. The convictions must be reversed and the cases remanded for a new trial. So ordered.