KIEV, Ukraine — The catapult that went up on a central street of this city on Monday was a clear sign that the protests that have been going on here for more than two months were taking a darker turn.

About 10 feet tall, the catapult was piled with bags full of cobblestones to send whistling into the ranks of the police. Men in masks hovered around it but did not let loose, apparently fearful of hitting protesters, given the large crowd nearby.

Clashes between demonstrators and the police continued Monday after fighting broke out the day before between riot police officers and protesters who had gathered to object to new laws limiting public assembly. A struggle began for a small swath of a side street near Independence Square, the center of the protest.