HONG KONG — Pro-democracy demonstrators in Hong Kong suffered a setback on Monday, when their attempt overnight to besiege government offices collapsed and the police thrust into the protesters’ biggest street camp.

The reversal came after a night of seesaw clashes in the political heart of the city, ending weeks of anxious calm at the protesters’ main street camp and throwing into question how much longer the Hong Kong government would tolerate hundreds of tents only a stone’s throw from the city’s administrative and legislative complex.

Fear rippled through the protest camp, with some student leaders defending the decision to escalate the confrontation with the police, and others wondering whether the protest leaders had made the right decision.

Many protesters wore masks and goggles, worried that the police would use pepper spray.

“The police have never gotten so close to the heart of our camp,” said Augustine Chung, a 24-year-old employee of a nongovernmental organization who was among the protesters. “I can only hope the student leaders know what to do next.”