At least 15 people were killed during protests this week in Nicaragua, raising the death toll of the nation’s six-week political uprising to about 100, human rights activists said Thursday.

The latest killings took place at a march on Wednesday attended by hundreds of thousands of people on Nicaragua’s Mother’s Day. It was held to honor the mothers of students killed at previous rallies.

A leading Roman Catholic bishop called it a “massacre,” and the Episcopal Conference of Nicaragua, which had been mediating a national dialogue between the opposition and the government, abruptly ended the peace talks.

“How can you dialogue with your assassins?” said Gonzalo Carrión, a lawyer at the Nicaraguan Center for Human Rights, who said the police had opened fire on protesters. “This was the biggest rally yet. It was a homage to mothers who lost their sons at rallies in April and May, and they wound up adding 15 more mothers to that list.”