More than 10,000 Nicaraguans peacefully protested against the government here today with placards and chants calling for opposition unity and the fall of President Anastasio Somoza.

For the first time since widespread violence began a year ago, erupting into a brief civil war last September, black-and-red flags of the Sandinista Liberation Front guerrillas were carried openly through Mangua's streets.

Some opposition leaders had anticipated a crowd approaching 100,000. Many people reportedly stayed home out of fear that violence would break out between demonstrators and government troops.

Not a single uniformed National Guard soldier was seen in the vicinity during the demonstration. As the crowd dispersed at dusk, however, scattered shots briefly rang out, apparently fired by groups of youthful demonstrators walking home. Several soldiers in jeeps quickly arrived and shot into the air. One youth reportedly was wounded. The incident appeared to end quickly.

Somoza had said the government would not interfere with the protest, for which opposition politicians were granted a permit Tuesday.

Tight National Guard security that began Monday in and around the city -- including military checkpoints and heavily armed patrols -- appeared to have been lifted by midday.

Early yesterday evening, Luis Medrano Flores, a 37-year-old labor leader, was shot and killed by unidentified gunmen as he distributed leaflets advertising the demonstration.

In pre-dawn hours this morning, National Guard troops were seen pasting antiguerrilla posters on the church where the demonstration was scheduled to begin and along the march route.

The son of an opposition leader and three other men who arrived at day-light to tear down the signs were arrested by soldiers but released several hours later.

Numerous National Guard arrests of young men in outlying cities and slum areas around Managua have been reported throughout the week.

As the demonstration began, a Sandinista-National Guard clash was reported on Nicaragua's southern border with Costa Rica. The intensification of such attacks in recent weeks, along with heavy guerrilla fighting in the north, has led to growing belief here that a long-awaited Sandinista offensive may soon begin.

Today's demonstration commemorated the first anniversary of the slaying in downtown Managua of opposition newspaper publisher Pedro Joaquin Chamorro. His death provided a catalyst for growing anti-Somoza sentiment.

In banner headlines today, Chamorro's newspaper La Prensa declared that the Somoza "regime killed his body, but Pedro Joaquin lives," and pledged that "Nicaragua will once again be a republic." It contends that Somoza's rule is virtually a royal dynasty.

The newspaper also noted, as it has in a front-page box every day since the murder, that the killers remain unpunished. Four men who allagedly gunned Chamorro down as he was driving to work have never been brought to trial, although they were arrested quickly. An alleged ringleader of the assassination plot remains unidentified.

In Chamorro's memory, several thousand Nicaraguans crowded into a church in west Managua for a mass celebrated by Archbishop Miguel Obandoy Bravo, who last year joined political groups and guerrillas calling for Somoza's ouster.

At the end of the mass, those inside joined thousands more representing political civic and labor groups gathered outside. Each group carried an identifying banner in the march to the cemetery where Chamorro is buried.

Some groups carried Sandinista banners, and youths armed with cans of spray paint covered walls and cars parked along the route with guerrilla slogans.