BANGKOK (Reuters) - Troops fired into the air as Thai anti-government protesters stormed the country's interior ministry on Sunday after Prime Minister Abhisit Vejjajiva declared a state of emergency in the capital.

About 50 protesters broke through security at the interior ministry in Bangkok while Abhisit was in the building, but the prime minister escaped by car, a TV channel said.

Reuters journalists at the scene said soldiers initially made no effort to stop the protesters from entering the building but later fired into the air to prevent more from joining them.

Red-shirted protesters were also gathering at the capital's police headquarters while a crowd at Government House had grown to over 4,000.

The latest unrest comes a day after protesters stormed the venue of a summit of Asian leaders, forcing the event to be canceled and seriously undermining confidence in the government.

In a weekly address to the nation, Abhisit said arrest warrants were being drawn up for those responsible for the unrest.

Police said they had already arrested Arismun Pongreungrong, a popular singer prominent in the summit assault, and were holding him at a police station north of Bangkok.

"In the current situation, what I have to do is to bring peace to the country, bring back governance and have a process of political reform," Abhisit said.

POLITICAL HUMILIATION

Abhisit suffered a political humiliation when the summit he had presented as a sign of the country's return to normality had to be canceled after red-shirted supporters of former Prime Minister Thaksin Shinawatra broke into the venue, sending Asian leaders fleeing by helicopter.

Thaksin's supporters say Abhisit only became premier because of a parliamentary stitch-up engineered by the army. They want new elections, which they would be well placed to win.

The events will pile more pressure on an economy teetering on the brink of recession, especially if foreign tourists are put off by the scenes of chaos.

Thai financial markets are closed until Thursday for a holiday. After months of falls, many Asian stock markets have rallied in recent weeks but the Thai market has been held back by the political unrest and is flat on the year.

Patareeya Benjapolchai, president of the Stock Exchange of Thailand, was concerned.

"It's really up to the government now how it manages the situation within this five-day break. What happened was a loss for the country. The ASEAN summit was supposed to be a step-up for our economy," Patareeya told Reuters.

Newspapers were outraged both by the pro-Thaksin supporters' insult to foreign leaders and by the government's inability to put proper security in place. Police and soldiers put up little resistance as the demonstrators marched toward the summit hotel.

"Yesterday was a truly shameful day for our country, which had its international image destroyed," the Bangkok Post said in a front-page editorial.

(Additional reporting by Kittipong Soonprasert, Viparat Jantraprap and Vithoon Amorn; Editing by David Fox)