The city teachers’ union is launching its first public protest against the Bloomberg administration in months. The president of the union, Randi Weingarten, organized tonight’s candlelight vigil outside the Department of Education’s downtown headquarters to protest a new city project to target bad teachers.

Ms. Weingarten said she received an “avalanche” of calls from concerned teachers about the new Teacher Performance Unit, a group of lawyers that will help principals fire teachers they think are not doing a good job, after it was announced last week. Ms. Weingarten condemned the plan as a “gotcha” unit that fixates on punishment rather than improvement.

A deputy schools chancellor, Christopher Cerf, defended the plan. “When everybody agrees that effective teaching is the single most important consideration in closing the achievement gap, it’s very difficult for the union to defend its position,” he said.

Whitney Tilson, the co-founder of an education advocacy group, Democrats for Education Reform, characterized the vigil as an attempt by Ms. Weingarten to pacify her members, not a serious challenge. “Let them have their vigil, and then sanity will return,” he said.

Some union members have complained that Ms. Weingarten has not taken on the city more strongly.

A retired teacher who has led the complaints, Norman Scott, said teachers frustrated with the union’s stance have been gathering monthly.

Mr. Scott said the group has also been meeting at the steps of the Education Department headquarters.