RIGA, U.S.S.R., NOV. 18 -- Several hundred armed militiamen today blocked Latvians from protesting Soviet rule, while others kept silent vigil in the rain-drenched streets of the Baltic city late into the evening.

After dusk, several Latvian youth groups attempted to break a tight Soviet police barricade around the Freedom Monument in the center of Riga, according to reports circulating here, leading to some clashes and arrests. In a telephone interview tonight, Latvian dissident Ints Calitis said that the extent of the confrontation was "not major."

The heavy presence of uniformed militiamen, plainclothesmen and civilian vigilantes in Riga's streets, coupled with the shrill campaign by Soviet authorities warning against the protests, signaled a sharp official turn against open demonstrations and another sign of conservative backlash against Communist Party leader Mikhail Gorbachev's policy of glasnost, or "openness."

Major protests that unfolded here in June and August, when support for Gorbachev's drive seemed to peak, were conducted without such heavy-handed official interference.

After Latvians began laying flowers this morning at the Freedom Monument in central Riga, in honor of the day in 1918 that Latvia declared its independence, Soviet police barred access to statue by forming a human ring 600 yards away from it. As evening fell, police widened the barrier and then dispersed at about 10 p.m.

Latvia, declared a free autonomous region on Nov. 18, 1918, was placed under Soviet rule in 1939, when Nazi Germany and the Soviet Union signed a nonaggression pact. Under the Ribbentrop-Molotov pact, Nazi German leader Adolf Hitler promised not to wage a war against the Soviet Union, while Kremlin leader Joseph Stalin gained control of Latvia, Lithuania and Estonia.

Although there were no major clashes today, Janis Barkans, the head of the Riga-based human rights monitoring group Helsinki '86, was put under house arrest.

The detention of Barkans, who is one of Latvia's leading activists, climaxed an all-out effort to thwart populist plans to celebrate the anniversary.

"This holiday should be celebrated," Calitis said in an interview here today, "but the authorities don't know how to accept that. They're not prepared for a dialogue, so they resort to the old methods."

Hundreds gathered during the day in a silent celebration of the holiday, the first widespread public acknowledgement of the anniversary date in nearly four decades. People braved drizzle to stand in clusters at the edge of the police barricade, staring sadly at the towering monument in the distance.

"I am here in solidarity," one woman said. "We should be allowed to demonstrate. We should have an independent Latvia."

Soviet officials here, climaxing a major campaign against the commemoration of Latvian independence day, sponsored a demonstration at 4 p.m. to protest a recent U.S. congressional resolution honoring the occasion. About 2,000 attended, carrying banners charging Washington with interference in Latvian affairs. "Leave us in peace," read one poster.

"The decision of the United States Congress on the pseudo-historic date of Nov. 18 as 'Latvian Independence Day' is not only a flagrant interference in the internal affairs of Latvians but also an insult to their national dignity," Riga Mayor Alfred Rubiks said in an interview with the official Soviet news agency Tass.

In all, an estimated 700 to 1,000 Soviet policemen circulated in the streets of Riga, buttressed by a civilian vigilante force that appeared to total more than 3,000.

A campaign against the demonstration has been building up for several weeks.

The campaign has included television interviews of local officials warning against particpation. Monday evening an interview was aired with Linards Grantins, a former leader of the Helsinki '86 group who was jailed in June.

In the interview, apparently taped in jail, the dissident urged Latvians not to participate in a demonstration today. Other dissidents interviewed today said it is "unlikely," that Grantins gave the interview voluntarily.

In addition to the warnings in the media, party officials have cautioned Latvian schoolchildren and older students against going to the Freedom Monument.

On June 14, Latvians gathered in Riga to mourn the victims of Stalin's purges.

On Aug. 23, the anniversary of the Soviet annexation of Latvia, an estimated 7,000 people held a peaceful protest. In both cases, police permitted the demonstrations.