MASAYA, NICARAGUA, FEB. 9 -- A mothers' demonstration against the military draft turned into a rock-throwing melee between pro- and anti-Sandinista political groups yesterday, police and participants said today.

The riot was the first violent episode in what both government and opposition leaders expect will be a period of sharpened political confrontation in Nicaragua, after the leftist Sandinista government lifted a six-year-old state of emergency last month as part of a regional peace plan and the U.S. House of Representatives voted last week not to give military aid to the rebels known as contras. Some government officials said they feared that loosening political controls could open the way for outbursts of protest because of the severe economic crisis gripping Nicaragua. Since the emergency was lifted on Jan. 16, there has been an increase in public opposition protests.

The military draft, first imposed by the Sandinistas in 1983, has been a continuing cause of friction between the government and the opposition, and many young men have tried to avoid the obligatory two-year service.

No deaths or serious injuries were reported in the clash. But antigovernment demonstrators shattered many windows of Masaya's main police station, burned two cars and battered a Sandinista Youth headquarters with rocks. At least six opposition protesters were arrested, the Interior Ministry said.

The turmoil began yesterday morning when Army recruiters began rounding up youths from homes and streets in a new draft drive in Masaya, 20 miles south of the capital.

By midafternoon about 50 irate mothers gathered outside the police station, where the recruits were being held, clamoring for their release.

The mothers' protest dissolved without incident at dusk, a participant, Lilian Urbina, said. But a few hours later several hundred more militant opponents of the government took to the streets again.

At the same time the ruling Sandinista National Liberation Front organized a march of its followers, armed with sticks and rocks.

The two sides clashed in the street in front of the police station in a "pitched battle" that ebbed and flowed until after midnight, a Sandinista demonstrator, shopkeeper Arturo Cardenas said.

"We wanted to respond to their provocation by showing them the force of our revolution," Cardenas said.

Some opposition protesters were carrying small pistols, and several shots were fired into the air.

The Sandinista police refrained from intervening during the scuffles, witnesses said.

Interior Minister Tomas Borge was in Masaya last night monitoring the events.

During the night State Security police went to the homes of several opposition leaders in Masaya to arrest them.

This morning shaken, sobbing relatives of Humberto Urbina, a well known member of the opposition Social Christian Party, said he had been watching television at home when the riot started last night. But the police came to arrest him in the early hours of the morning.

Tension remained high throughout Masaya today.

Police went to the home of 16-year-old Sergio Calero this morning with a warrant for his arrest in connection with last night's vandalism. But an angry anti-Sandinista crowd gathered around the front door.

When Calero's father came out, many on the street shouted, "Don't give them your son!"

Finally the police left without the boy.

In a communique the Interior Ministry blamed the riot on agents provocateurs "guided by the CIA." It said some were former members of the late dictator Anastasio Somoza's National Guard who were recently released from prison under a Sandinista pardon.