SOFIA, Bulgaria (AFP) - At least seven Bulgarian protesters and two policemen were injured in clashes late on Tuesday as demonstrators blockaded the parliament building, the first violence seen in 40 days of massive anti-government rallies.

Several thousand protesters blocked the parliament building from the early evening, preventing ministers, lawmakers and other officials from leaving after budget revision discussions.

Among those stuck in the building were three ministers from the embattled Socialist-backed technocrat cabinet along with some 30 lawmakers, journalists, trade union and employers groups members and parliament staff, police said.

The tensions rose shortly after 10pm (3am Singapore time) when police tried to evacuate some of them by bus.

About 2,000 protesters cordoned off the building shouting "Mafia!" and "Resign!" and stopped the vehicle.

Stones flew, breaking several windows on the bus, as police in anti-riot gear tried to push the protesters back to make way for the vehicle.

Police asked the crowd, which included many women, to back off but the protesters responded with cries of "no violence", "murderers" and "why do you beat our children?" After several attempts to leave the area, the bus returned to the building.

Local television broadcast the clashes live, showing several injured protesters and one policeman.

Four people, including one with concussion, were treated in hospital. Three more and two policemen were taken to the military medical academy with various head injuries, hospital officials told state BNR radio.

"There are lightly injured people and policemen and there have been arrests... No lawmakers have been hurt," Interior Minister Tsvetlin Yovchev told private Nova television but refused to give more details.

He said that police were considering whether to make another attempt to evacuate those unable to leave the parliament or to leave them inside the building for the night.

Up to 10,000 people have taken to Sofia streets every evening since June 14 in the European Union's poorest country, demanding the resignation of the new technocrat cabinet that took office in May after inconclusive snap elections.

Public discontent was initially sparked by the appointment of a media mogul as head of the country's powerful security agency.

But it later grew into wider anger against the cabinet and politicians in general seen by many as "corrupt" and too easily swayed by powerful "oligarchs".

The protests had previously been peaceful, with people marching along Sofia's boulevards with their families, friends, toddlers, dogs and bikes.

"For the first time since the beginning of the protests we witnessed today tension and attempts for provocations. I appeal for restraint from any acts leading to an escalation of the tension and the breaking of public order," President Rosen Plevneliev urged in a statement on Tuesday night.

He appealed for both protesters and police to refrain from violence and to make every effort to keep the protests peaceful.

Mass protests early this year saw eight people torch themselves, six of whom died, prompting the previous conservative government of Premier Boyko Borisov to resign at the end of February.

"I insist that the government resign immediately. This is the only way to appease the people," Mr Borisov said late on Tuesday.