International observers refused to give a "clean bill of health" to the general election in Bulgaria at the weekend, which saw the centre-right opposition roundly defeat the ruling Socialists.

The Organisation for Security and Cooperation in Europe still said Sunday's vote was "competitive and generally well-run" and that the outcome reflected "the will of the people."

The OSCE and the Parliamentary Assembly of the Council of Europe (PACE) sent monitors for the first such observer mission to a general election in a European Union member state.

According to official preliminary results, there was a sharp swing to the right, with the centre-right opposition GERB party of Sofia mayor Boyko Borisov defeating Prime Minister Sergey Stanishev and his Socialists, who have ruled for the past four years.

Turnout was high at 60.2 percent of the 6.8 million eligible to vote.

With the vote count almost completed Monday, GERB won 39.7 percent, with up to 116 seats in the 240-seat parliament.

The Socialists had 17.7 percent and 40 seats, only half of their previous total of seats.

PACE was "not ready to give a clean bill of health" to the election, mission chief Tadeusz Iwinski told a news conference.

"We are unhappy about the evident lack of public confidence in the democratic process and an overall voter cynicism exemplified in the broadly admitted practice of selling and buying votes," Iwinski said.

The mission would propose that PACE re-open monitoring procedures for Bulgaria, he said.

Since the collapse of communism, so-called vote-buying -- where small sums of money, food and other gifts are offered in return for support -- has been common in Bulgaria, particularly among the impoverished Roma population.

The government introduced heavy fines and prison sentences for those convicted of buying or selling votes. "But this did not stop the pervasive and persistent allegations of vote-buying," said OSCE mission chief, Colin Munro.

Neither the OSCE nor the PACE had many observers at polling stations. They relied mainly on media reports and discussions with diplomats, journalists and party and electoral commission members, Munro and Iwinski said.

Another moot point was the number of suspected criminals, some already on trial, who ran for parliament.

Under Bulgaria's immunity laws, court proceedings are automatically suspended for political candidates until the end of their term in office if elected.

The so-called Galev brothers, Plamen Galev and Angel Hristov, are under investigation for racketeering, but were released from custody as soon as they registered as candidates.

Neither of them won a seat.

In the campaign, Borisov, a former firefighter and bodyguard with a karate black belt, came to personify "the strong hand" many people believe Bulgaria needs to combat political corruption and the economic crisis.

He vowed "to restore the European Union's trust in Bulgaria and unblock the millions of aid for farming and infrastructure," frozen last year over concerns about the country's corruption problem.

Such was the scale of GERB's success, which confounded pre-election polls, Borisov could form a coalition government with one of two small right-wing formations that also entered parliament.

They are the Blue Coalition with 15 seats; and the Order, Lawfulness, Justice party with 10 seats.