With Thailand reeling after its bloodiest street violence in years, red-shirted protesters have vowed to continue their campaign against Prime Minister Abhisit Vejjajiva until he dissolves the parliament and calls fresh elections.

Twenty or more people were killed during anti-government protests in the Thai capital Bangkok on Saturday as soldiers and police fought pitched battles, firing tear gas and bullets at demonstrators, who in turn responded with rocks and Molotov cocktails.

After more than two hours of fierce clashes, the soldiers pulled away from the area.

Five hundred more people were wounded, according to Thai hospital officials, who saw the aftermath of the bloody riots.

The Red Shirts, who are mainly from poor rural areas are aligned to former PM Thaksin Shinawatra, and have gathered in Bangkok to complain about what they see as an illegal government.

They have claimed the current government only came to power because of a 2008 court ruling which ousted the government allied to Thaksin.

Thaksin himself was ousted in a military coup.

The protesters have now called on King Bhumibol to intervene.

The King has been on the throne for more than 60 years and has been unwell, but is revered by the entire nation.