MOSCOW, FEB. 16 -- Demonstrations planned in Lithuania today to commemorate a two-decade-long period of independence were blocked by a heavy Soviet police presence and harsh warnings from local Communist authorities, according to Lithuanian dissident sources reached by telephone.

Patrols of uniformed policemen and civilian auxiliaries have been circulating in the Lithuanian capital Vilnius since the weekend, stifling any spontaneous protests, said the sources, who asked not to be identified for fear that they would be punished.

The sources said that the mood in Vilnius was calm, however, with no visible scuffles or conflicts in the streets.

Nationalist activists had urged Lithuanians to place flowers in several places to mark the small Baltic republic's independence from Russia, which was declared 70 years ago and maintained until the Soviet Union occupied and annexed it in 1939 and 1940.

But local authorities reportedly preempted the demonstrations with police patrols and other measures, including placing some activists under house arrest.

Police guards were placed in front of the house of activist Nijole Sadunaite, the official news agency Tass reported in a dispatch from Vilnius today. The guards explained that they were there to protect Sadunaite's safety, Tass said.

Communist Party activists last week canvassed schools and factories throughout the republic, according to dissident sources, reading warnings from the Lithuanian party's Central Committee against participation in any demonstrations. Teachers and factory foremen were also told they would be held personally responsible for the participation of their charges in protests, the sources said.

Soviet authorities also placed heavy restrictions on press coverage of the events in Vilnius.

The Soviet Foreign Ministry invited a handful of western journalists to Lithuania for two days of meetings with local party leaders and other officials. Other western journalists applying for permission to visit Vilnius were told that the area was temporarily closed.

Telephone lines into Lithuania were also heavily monitered. The telephones of at least a dozen dissidents were disconnected Sunday morning.

Today's events highlight the problems of Baltic nationalists who are waging a campaign for the independence of the three Soviet republics along the Baltic Sea -- Estonia, Latvia and Lithuania.

The republics were occupied and annexed by Soviet leader Joseph Stalin in 1940, a year after he signed a secret agreement with Nazi Germany.

Under the pact, Germany gave up its claims to the republics to the Soviet Union.

The United States has never recognized the agreement.. President Reagan and several members of Congress sent messages asking the Kremlin leadership to permit commemoration of the anniversary in Lithuania.

An official demonstration was organized by the party in Vilnius yesterday afternoon to protest what they called Reagan's interference in local affairs.

Today an official delegation from Vilnius attempted to meet with U.S. Ambassador Jack Matlock in Moscow to protest the U.S. comments, Tass reported, but Matlock refused to see them.

Independence day demonstrations in the heavily Catholic cities of Vilnius and Kaunas began Sunday, when hundreds of residents reportedly flooded into the churches.