MOSCOW, FEB. 25 -- Soviet troops have been sent into the Armenian capital, Yerevan, to calm a massive nationalist-inspired demonstration there, according to witnesses, but the crowds and the protests have shown no signs of subsiding as they begin their second week.

Armen Karen, an Armenian activist, said in a telephone call from Yerevan today that Soviet soldiers had been dispatched to scattered parts of the city to supplement the local militia deployed throughout the capital to control the protests. But neither the armed militia nor the military has used violence against the demonstrators, Karen said. Two other Armenians arriving here from Yerevan confirmed the accounts.

The use of Soviet military units and the presence of four members of the Soviet leadership in the area indicated that the demonstrations are occupying the attention of the key party officials in Moscow.

The demonstrators formed a committee yesterday to negotiate with Soviet authorities and continued to give speeches in the streets of Yerevan all day today, Karen said, spurning appeals by Kremlin leaders for calm.

Since Armenians in Yerevan first took to the streets to back an appeal for the heavily Armenian region of Nagorno-Karabakh, located in the neighboring republic of Azerbaijan, to be made part of Armenia, the city's streets have filled with growing crowds, Karen and other Armenian activists said.

Estimates of the number of demonstrators ranged widely, with some as low as 25,000 and others as high as 200,000.

Soviet Foreign Ministry spokesman Gennadi Gerasimov confirmed in a briefing yesterday that some demonstrations had taken place but he declined to give an official estimate of the number of participants.

Yerevan has been temporarily closed to foreign journalists, making an independent estimate impossible.

The protest by Armenian nationalists, now said to be taking place in several cities, broke out earlier this month when Armenians in Nagorno-Karabakh took to the streets to demand that the predominantly Armenian region, incorporated into the republic of Azerbaijan by a 1923 Kremlin decision, be reunited with Armenia.

The protest is a climax of longstanding tensions between Azerbaijanis and Armenians in Nagorno-Karabakh. Demonstrations spilled over into Yerevan Friday, witnesses said, after rumors began to reach the Armenian capital that violence and tanks had been used to quell the protests in Stepanakert, the capital of Nagorno-Karabakh, and that some deaths had occurred.

Reports of deaths during the protests could not be confirmed. Soviet Deputy Procurator Alexander Katusev, now engaged in follow-up work in Stepanakert, rejected the reports in an interview published by Tass today. "All sorts of rumors and inventions are groundless," he said.

"There was a rumor about a bandit attack by traffic militia officers on a car owned by a resident of Stepanakert," he said, adding, "the rumor proved false."

"There was the rumor about the murder of 60 Armenians, although not a single person of Armenian nationality has fallen victim to murder," he added.

In an attempt to bring calm, local party officials ousted party chief Boris Kevorkov yesterday and replaced him with Genrikh Pogosyan, an ethnic Armenian.

Nonvoting Politburo members Georgi Razumovsky and Pyotr Demichev, dispatched to the region early this week to quash the demonstrations, have appealed to local party leaders for support against the demonstrators.

In Yerevan, Vladimir Dolgikh, also a nonvoting Politburo member, and Anatoly Lykyanov, a Central Committee secretary, have been holding negotiations with local dissidents and party officials to bring calm to the area.