For the first time, voters will cast two separate ballots, the first for electing directly one deputy for each of the 31 constituencies and the second to fill the other 209 deputy seats, allotted on a proportional basis.

According to eve-of-ballot opinion polls published on Saturday, the centre-right opposition GERB party will win the vote with 28 to 34 per cent support, while the ruling Socialists party is credited with 19 to 23 per cent of the vote.

Both parties stand no real chance of gaining a majority in the 240-seat parliament to form a stable government even with support from their two preferred partners.

Corruption claims

Numerous vote-buying allegations have already marred the electoral campaign, prompting the interior ministry to provide reinforced police presence in regions where the practice is suspected to occur.

Political parties in the poorest European Union state 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.

Police and the national security agency said they had launched a campaign across the Balkan country to prevent vote buying and made at least five arrests so far.

Police confiscated hundreds of identity cards, lists of names, computers and cash.

The non-government group, Union for Business Initiative, said earlier this week that drug dealers offered free drugs in Bulgaria's second biggest city of Plovdiv in return for votes, while police were investigating a case in the city of Pazardzhik of Roma people having their overdue water bills paid.

In an address to the nation, Sergei Stanishev, the prime minister, called on Bulgarians not to sell their votes and warned politicians, parties and vote buyers they would be punished if found guilty.

"These disgusting practices inflict terrible scars on our democracy. But the most scary thing is that in return for a pathetically small amount of money, you put your own future and that of your children and families at stake," Stanishev said.

"I have given categorical instructions to police and the national security agency to ... send to court everyone who takes part in this crime."

In April, parliament doubled the maximum jail term to six years for those who organise vote-buying schemes and five years for vote buyers themselves.