Opinion polls suggest Prime Minister Simeon Saxe-Coburg, the country's former king, will pay the price for his government's austerity measures. One published poll on Thursday gave the opposition Socialists a 14% lead.
A change of power would continue a trend in Bulgaria over the last 15 years in which no government has been re-elected, mainly because of tough reforms all have had to implement since the fall of communism.
The Socialist Party has not formed a government since 1997.
In a nationwide broadcast on Thursday, Saxcoburggotski called on Bulgarians to give him a chance to carry on with his reforms, saying the remaining 18 months until the country is admitted into the EU are of "decisive importance".
During his term in office, Bulgaria, a former Soviet ally, has become an enthusiastic supporter of the US-led campaign against terrorism, and has a 460-member force in Iraq.
But the deaths of 13 Bulgarian soldiers in Iraq since 2003 prompted public anger.
Government achievements include 5.6% economic growth in 2004, a record $2.5 billion in foreign investment, single-digit inflation and a drop in unemployment to less than 12% from 18% in 2001.
PM Saxe-Coburg (L) may pay a
high price for austerity steps
But with an average monthly salary of $255, Bulgarians are in little mood for celebration.
"Due to mistakes made by the government some of the reforms were delayed and not all people felt the effect as swiftly as I hoped," Saxe-Coburg acknowledged, adding that the government was unsuccessful in the fight against poverty, corruption and organised crime.
No party is likely to gain a sufficient majority to govern alone, which would force the Socialists to look for a coalition partner.
Over 6000 candidates from 22 political parties and coalitions will compete for the 240 seats. Voting stations for the 6.7 million eligible voters open as of 0300 GMT and final results are expected late Monday.
Besides the incumbents and the Socialists, at least four more parties are expected to win seats in the chamber: the United Democratic Forces, expected to win some 10% of votes, the mainly Turkish Movement for Rights and Freedoms with 8%, the Democrats for Strong Bulgaria - 6% and centre-right coalition Bulgarian Popular Union with 6%.
Surprisingly, the ultranationalist Attack party - according to the latest polls - could reach the 4% threshold needed to win parliamentary seats.