“The situation of the last two or three days gave me no choice,” he said, referring to his decision to use force. “I had to enforce the law. If the government had done nothing, it would have shown its weakness.”

Constrained by strict rules of engagement, soldiers used mostly tear gas and rubber bullets, Mr. Abhisit said.

He conceded the accuracy of protesters’ claims that live ammunition had also been used, but he said it had been fired only into the air or in cases of self-defense.

The army said that 60 soldiers had been wounded, including two who had been shot. There were several reports that protesters had used explosive devices, and Mr. Abhisit said that these blasts had caused many of the injuries.

Image Thai soldiers aided an antigovernment protester who was overcome by tear gas during street battles in Bangkok on Saturday. Credit... Agnes Dherbeys for The New York Times

The government-run Erawan Medical Center said that at least 18 people had been killed and 678 wounded. It said four of the dead were soldiers and the rest were civilians.