ISTANBUL — Late one night last summer, at the height of antigovernment demonstrations sweeping Turkey, a group of protest leaders rushed to the home of Prime Minister Recep Tayyip Erdogan in Ankara, the capital, to negotiate a solution to the growing crisis.

They came away with a tentative agreement, but it was never accepted by the rank-and-file protesters, and so the movement was later crushed by the water cannons and tear gas of Mr. Erdogan’s police force.

Then last month, one of those leaders, Eyup Muhcu, was summoned by a local prosecutor and interrogated as part of a spreading investigation of those who led the protests. “There is no concrete charge, yet we were called in to give official statements,” said Mr. Muhcu, an architect and a member of the Taksim Solidarity Platform, a group of activists that played a central role in the demonstrations.

“For what?”

For the government, the answer seems clear, Mr. Muhcu said: to silence the opposition.

“It has come to a point where members can’t even tweet without fear of being investigated for their thoughts,” said Mr. Muhcu, one of the few activists still willing to offer a public critique of the government.