The party of Bulgaria's ousted premier Boyko Borisov came first in tense elections Sunday but fell short of a majority, exit polls showed, setting the scene for political stalemate and fresh protests.

Three months after the biggest demonstrations in years prompted the former bodyguard to tender his government's resignation, Borisov's GERB party won between 29.6 and 32 percent of the vote, exit polls showed, based on a count of votes from two percent of constituencies.

The socialist BSP party came second with between 25.6 and 26.2 percent, followed by Turkish minority party MRF on 9.9-13.4 percent and the ultra-nationalist Ataka on 7.0-8.5 percent.

It remained unclear yet if a fifth party -- most likely the far-right National Front for the Liberation of Bulgaria -- had passed the 4.0 percent threshold to win parliamentary seats in the European Union's poorest country.

The tight vote made it impossible for pollsters to say if GERB could muster a slim majority together with Ataka -- which used to back their previous majority cabinet but turned against them as the vote drew near.

"There is no way for Ataka to back GERB," snapped Ataka's flamboyant leader Volen Siderov late Sunday.

"Bulgaria needs stability and if parties are responsible, they should back even a minority cabinet," GERB's campaign manager and ex-interior minister Tsvetan Tsvetanov said.

Borisov has not made any comment so far.

The Socialists and their previous coalition partner MRF were also close to a majority in the 240-seat parliament, the polls suggested.

"Slowly and painfully Bulgaria is getting rid of GERB and this is a healing process. It is clear that GERB will not be able to form a government and this is good for Bulgaria," Socialist leader Sergey Stanishev said.

He urged "quick talks with all parties except GERB on a program for taking the country out of the crisis ... and calming social tensions."

BSP said earlier they favored a crisis-fighting cabinet of technocrats with broad support.

"This is good-bye for GERB. They will not govern the country," Gallup analyst Kancho Stoychev said, adding that a scandal-ridden campaign and allegations about vote-rigging had put GERB in "total isolation."

"There is a big risk there will be a deadlocked parliament," added another analyst, Ognyan Minchev.

Failure to form a government would mean new snap elections in the autumn and continuing with the current caretaker cabinet.

About 50 percent of the 6.9 million Bulgarians eligible to vote turned up at voting stations Sunday, the polls showed.

The first official results were not expected until late morning on Monday, with parliamentary seats not allocated until several days later.

Analysts say that with no clear result, Bulgaria was likely to see a repeat of the anti-poverty and corruption protests that rocked the country over the winter and forced Borisov to throw in the towel.

Some 150 protesters clashed with police as they tried to storm the main election press centre shortly before parties were due to hold their first official news conferences late Sunday.

Stones were hurled at police, who used batons and shields to push back the crowd that shouted "Mafia!" and "Annulment." There were no reports of injuries.

"The vote is on track to repeat the previous parliament of the oligarchy and mafia that we forced out. We'll have barricades this time not just protests," one of the organizers of the rallies, Angel Slavchev, said.

Bulgaria's winter of discontent saw seven people set themselves on fire, six of whom died.

Six years after joining the European Union, almost a quarter of Bulgarians live below the official poverty line, living standards are falling and poverty is on the rise.

A prolonged stalemate would damage the country's fragile economy, which grew just 0.8 percent in 2012 as unemployment soared up to almost 20 percent, according to unofficial estimates.

Many voters feel that politicians spend too much time bickering and lining their own pockets and not enough tackling the country's economic problems, corruption and inequalities.

Further poisoning the atmosphere were a mudslinging scandal about illegal wiretaps and allegations of vote-rigging during the election campaign, including on Saturday the discovery of 350,000 unaccounted-for ballot papers at a printing firm whose owner is reportedly close to Borisov's party.

Stanishev said the discovery was "preparation for the total falsification of the elections," calling it a "scandal unseen in Bulgaria so far."

Five opposition parties -- excluding GERB -- have commissioned an independent vote count by an Austrian agency, and it is possible that the official results will be challenged.

Partial results from this agency, based on a parallel count in 164 constituencies, also gave GERB the lead, with 30.1 percent, followed by the BSP with 26.1 percent, the MRF with 11.6 percent and Ataka with 7.8 percent.

The Organization for Security and Cooperation in Europe (OSCE), which sent its biggest monitoring mission to Bulgaria since its first post-communism elections in 1990, was due to report its findings on Monday. (AFP)