Anti-government protesters in Bangkok were given until 3pm local time (9am GMT) to withdraw from the area they have occupied as violence spreads to Chiang Mai.
The situation has worsened following the death of Major General Khattiya Sawasdipol who had defected to the side of the Red Shirt protesters and was shot in the head last week.
The attack led to renewed violence between the army and protesters, and concerns are growing the death will cause further fighting. So far 36 people have died in the protests.
Violence has spread to the northern city of Chiang Mai where hundreds of protesters marched in support of the Red Shirts and hand grenades were thrown at two banks.
Tourists are continuing to steer clear of the capital as the Foreign Office (FCO) continues to advise against all but essential travel to Bangkok while the British embassy remains closed.
The FCO is also advising against all travel to certain parts of the country and is urging visitors to exercise caution in other areas.