This is the latest in a polarising crisis that has been brewing for years. Two men represent the two camps: exiled former prime minister Thaksin Shinawatra and the new prime minister, Abhisit Vejjajiva.
The red-shirt protesters, described as Thailand’s urban and rural poor, accuse what they call an “establishment elite” of colluding to bring down two elected governments, including Thaksin’s.
They want parliament dissolved and new elections held, still angry that Thaksin was forced into exile after a coup in 2006.
They claim the new prime minister came to power illegitimately in a parliamentary vote engineered by the military, royalists and big business.
Parties allied to Thaksin have won the past three elections in landslides and would be heavily favoured to win a new one as well.
Observers say the violence makes any deal extremely difficult. The prime minister has vowed that security forces will not retreat but press on to end the mass protest.