Official preliminary results gave the former communists an estimated 124 seats in the 240-seat parliament, according to calculations made on the basis of 95 percent of the vote.
The Central Electoral Commission saw the anti-communist Union of Democratic Forces (UDF) trailing with 68 seats and gave its former ally, the People's Union, 19 seats.
Socialist leader Zhan Videnov said Bulgarians said voters had punished the UDF for "its unproductive, confrontational policy, its lack of competence, irresponsibility."
UDF leader Philip Dimitrov said his party's second place gave it few opportunities to control the situation.
Warning that Bulgaria's already slow reforms could be stopped, he repeated his appeal to other parties to combine efforts with the UDF "so that we can effectively pare any attempts at open restoration of communist models."
The results saw a turnout of more than 75 percent and gave the mainly ethnic Turkish Movement for Rights and Freedoms 16 seats.
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