SOFIA, BULGARIAE — BULGARIA'S former Communist Party lost its grip on power in multiparty elections Oct. 13 but will retain a strong presence in the new parliament, preliminary results showed.About 70,000 jubilant Bulgarians crammed a central Sofia square the night of Oct. 14 to celebrate the end of the former Communist Party's four-decade grasp on power. The Socialist Party, formerly the Communist Party, was overtaken by the noncommunist Union of Democratic Forces in the poll, according to estimates that gave the Democratic Union about 36.5 percent of the vote and the Socialists about 33 percent. "This is the first evening in the last 45 years of Bulgarian life in which the communists aren't in power," Democratic Union leader Philip Dimitrov shouted to the cheering crowd from a stage festooned with blue balloons. International observers said the voting was generally free and fair. The parliamentary elections were the first follow-up vote held by any of the East European countries that ousted their communist leaderships. The Movement for Rights and Freedoms (MRF), which represents Bulgaria's 1.5 million ethnic Turks, won up to 7 percent of the vote - and a potential power-broker role because neither of the two main groups secured a clear enough majority. State radio translated the estimates as 116 seats for the Democratic Union, 104 for the Socialists, and 20 for the MRF. With no one party having a clear majority, Western diplomats said, the Democratic Union was likely to seek support from the MRF. MRF leader Ahmed Dogan has ruled out collaboration with the former Communist Party, which persecuted the Turkish minority during the rule of Todor Zhivkov, who was ousted in 1989. Socialist leaders said they would accept the final election results but doubted any coalition excluding them could last. The Socialists won a majority in June 1990 elections, preserving the monopoly on power held by the Communists since 1946. The new parliament is expected to open next month, and Democratic Union officials said one of their first tasks would be to start a criminal investigation of a number of Socialist leaders for alleged economic abuses. Mr. Dogan said his movement would be prepared to present evidence about Socialist leaders' involvement in the persecution of Bulgarian Turks by the Zhivkov administration.