Hundreds of protesters ran into a side entrance of grounds as newly appointed Prime Minister Niwatthamrong Boonsongaisan was meeting election commissioners to discuss a July 20 election.

“The meeting is over, the prime minister is leaving. We cannot continue today,” said commission member Somchai Srisuthiyakorn.

Two men were killed instantly and three died later in hospital after unidentified assailants in a white pick-up vehicle fired gunshots and grenade launchers at a barricade outside a protesters' camp near Bangkok’s Democracy Monument.

The attack comes at a time of heightened tensions in a six-month political crisis that had already left 26 people dead and hundreds injured.

Thailand’s first woman prime minister, Yingluck Shinawatra, was forced from office last week by a controversial ruling in the country’s Constitutional Court relating to her demotion of a senior security official in 2011.

Pro-government Red Shirts have massed on Bangkok’s outskirts since the ruling which they said was part of an orchestrated campaign by influential establishment figures in Bangkok to force Ms Yingluck’s wealthy family to quit politics.

Red Shirt leaders have been urging their supporters not to react violently to the ruling, saying they fear an outbreak of violence could prompt the military to mount a coup.

The military, which has staged 18 coups or attempted coups since the 1930s, has repeatedly denied plans to intervene and urged political rivals to negotiate a settlement.

The protesters moved to Democracy Monument last weekend after months camping at Bangkok’s central Lumpini park.

There have been 80 violent attacks in Bangkok since protesters took to the streets last November.

One attack late last week was on the home of a Constitutional Court judge.

Attackers have often used grenade launchers.

Mr Niwatthamrong has vowed to push ahead with the election and promised a period of reform afterwards as a way of ending the unrest.

But the protest movements wants another prime minister and administration suitable to them installed before they end their campaign of destabilisation.

Elections held in February were annulled after anti-government protesters disrupted voting in some areas.

The election was boycotted by the opposition Democrat Party.

Thai authorities have issued warrants for the arrest of dozens of anti-protest leaders but police have not moved against them, apparently fearing this would provoke more violence.

The crisis is the latest chapter in a years-long struggle for power by rival groups of Thai elite, one backed by Bangkok’s middle class and royalist establishment and the other backed by rural masses who support the Shinawatra family, including Ms Yingluck’s elder brother Thaksin Shinawatra, a former prime minister and business tycoon who lives in exile to avoid a jail sentence for corruption.