He said witness accounts collected during a week-long inquiry in Tuzla, to which more than 6,000 refugees fled, left little doubt that atrocities had been widespread.
"These events require a thorough investigation. But I can say that what happened cannot be described as violations of human rights but as extremely serious violations on an enormous scale," Mr Mazowiecki said.
"What requires investigation is the scale and dimension, but we can say barbaric acts have been committed," he said. Mr Mazowiecki said the Bosnian Serbs had rebuffed repeated requests from his team to visit areas where thousands of Muslim males were believed to be detained.
Dutch UN peace-keeping troops, who were overwhelmed by the Serb onslaught on Srebrenica, said on Sunday after their return to UN mission headquarters in Croatia that they witnessed Serb soldiers executing and mutilating Muslims.
Two peace-keepers spotted nine Muslim men being hustled to a building for interrogation and then heard gunfire. They later found and photographed the corpses of the nine men with bullet wounds in their backs, a senior Dutch officer told reporters.
The Bosnian Serbs have denied committing atrocities in Srebrenica, but UN sources say that myriad and consistent accusations of summary executions, rape and other abuses made by the refugees appear credible.
Dutch soldiers could not confirm allegations that many Muslim women were raped by Serb soldiers after being removed from Serb-run expulsion convoys, but they added they had no way of monitoring the forced evacuation.
In Zagreb, UN spokesman Chris Gunness condemned the Serbs for "their inhuman and degrading treatment" of Srebrenica's inhabitants.