Bulgarians are voting in a parliamentary election expected to remove a Socialist-led coalition from power.
Sunday's vote is widely expected to hand power to the centre-right party of Boiko Borisov, the mayor of the capital Sofia.
Polls opened at 6am local time (0300 GMT) on Sunday and will close at 7pm (1600 GMT).
At least 6.8 million voters will be choosing between some 4,500 candidates from 14 political parties and four coalitions for parliament's 240 seats.
Latest opinion polls have suggested Sergei Stanishev, the prime minister, will find it hard to win the election.
Borisov is tipped to score an easy victory, but he may not get enough votes to form a government without seeking a coalition partner.
His party has been riding high on promises to punish corrupt officials and criminals.
Despite securing EU membership, Stanishev's government has been widely blamed for failing to improve the quality of life in the Balkan country of 7.6 million, the poorest member of the European Union.
Although unemployment stands at a relatively low seven per cent, opinion polls suggest more than a third of Bulgarians fear they might lose their job in the near future.
And while wage increases have pushed the average salary to $420, it remains the lowest in the 27-member EU.
On the eve of the election, Bulgarian police arrested at least five people on suspicion of vote buying.
The arrests on Saturday followed an appeal to Bulgarians by Stanishev not to jeopardise their future in the election.
The vote has been marred by widespread allegations of vote buying by virtually all political parties.
Political parties have been accusing each other of offering money, food and other basic commodities to mainly poor communities and minorities such as the Roma people in exchange for votes.
Bulgaria is also under pressure from the EU to combat corruption and organised crime to avoid more sanctions after Brussels cut its access to over half a billion euros in EU aid last year.