MOSCOW -- Soviet "Black Beret" troops seized the central telephone exchange in Vilnius, the Lithuanian capital, yesterday, severing the city's communications for more than two hours.
The commanders of the troops, who are responsible to the Ministry of Internal Affairs, said that they were looking for weapons caches and that they found explosives, a fuse and rifle ammunition.
Boris K. Pugo, the minister of internal affairs, said he knew nothing of the operation, but a ministry spokesman said later that it was justified and had been approved by a Soviet prosecutor.
No injuries were reported.
"This attack was not accidental," Lithuanian Vice President Caslovas Stankevicius said in a statement released by the republic's information office. "It was planned as a rehearsal for the overthrow of the legitimately elected government of Lithuania."
Mr. Stankevicius noted that before the Soviet-led invasion of Czechoslovakia in 1968, there were reports that arms caches had been found. Oleg D. Kalugin, a former KGB general turned pro-democracy activist, has said that the Soviet secret police planted those caches.
Lithuania has been the target of constant military pressure from Moscow since it became the first Soviet republic to declare its independence 15 months ago. Fourteen people were killed in January when troops using tanks and automatic weapons seized broadcasting operations in Vilnius.
In recent months, the Black Beret troops repeatedly have attacked and destroyed border posts and customs posts set up by Lithuania and the other Baltic republics, Estonia and Latvia. A customs post in the main railroad station in Riga, the Latvian capital, was ransacked yesterday by Black Berets, the Baltfax news agency reported.
Moscow officials routinely have disavowed prior knowledge of the Black Beret operations but have justified them after the fact.
Aurelius Katkevicius, an adviser to Lithuanian President Vytautas Landsbergis, told The Sun last night that he thought the telephone cutoff might have been designed to determine what backup communications facilities were available.
He linked the incident to another scare June 3, when troops were deployed around the parliament building in Vilnius, ostensibly to search for deserters from the Soviet army.
Mr. Katkevicius said by telephone from Vilnius after communications were restored that all the troop mobilizations appeared to be part of a coordinated plan to destabilize the political situation and prepare for the installation of a pro-Soviet puppet regime.
In recent weeks, hard-line forces in the Soviet Union have become outspoken. Conservative deputies in the Soviet parliament have demanded the ouster of President Mikhail S. Gorbachev, saying he has not moved strongly enough to save the Soviet empire from collapse as a result of independence movements in various republics.
KGB chief Vladimir A. Kryuchkov told the parliament in a closed session last week that the CIA was plotting the disintegration of the Soviet economy and state. In excerpts from his speech shown on Leningrad television, he suggested that Soviet reform was unfolding according to CIA plans and scoffed at hopes for heavy Western economic aid.
Soviet Prime Minister Valentin S. Pavlov has taken a similar stance, though not as uncompromising. Despite the heavy pressure from conservatives, however, Mr. Gorbachev has shown no sign of shifting to the right.
The Soviet president has met with economist Grigory Yavlinsky, who has developed a reform plan with Harvard University experts that calls for massive Western aid. Mr. Gorbachev has said that he will take a hybrid of the Yavlinsky plan and that of Mr. Pavlov when he meets with the leaders of the "Group of Seven" industrial nations in London next month.
Mr. Gorbachev seems to be willing to risk offending hard-liners and the KGB, if necessary, to maintain good relations with the West and with leaders of the republics, especially Russian President Boris N. Yeltsin.
Mr. Katkevicius said he doubted that Mr. Gorbachev was behind yesterday's telephone cutoff. He said he thought it more likely that hard-liners were planning a series of provocative actions aimed at embarrassing Mr. Gorbachev and preventing any agreement with the West in London.