Five explosions hit an area packed with heavily armed soldiers and studded with banks, office towers and hotels. Four were seriously wounded, including two foreigners, according to hospital officials.

The M-79 grenades, fired with a shoulder-mounted launcher into an area where hundreds of pro-government protesters were gathering, were the same type that hit troops during a bloody clash with protesters that killed 25 people on April 10.

Troops, many armed with M-16 assault rifles, have poured into the area since Monday to contain the anti-government "red shirts,” who have formed a barricade at an intersection leading into the bustling district also known for racy go-go bars.

The government said the grenades were fired from the red shirt protest area. Leaders of the red shirts, who have been demonstrating in Bangkok for nearly seven weeks seeking new elections, denied they were responsible.

Television footage showed blood splattered across sidewalks, office windows smashed and a chaotic scene as panicked residents carried the injured into nearly a dozen ambulances.

"It's worrying, seeing ambulances, people running away. The police and army don't seem to be in control," said Herman Koopman, a tourist from the Netherlands.

After the explosions, hundreds of pro-government protesters regrouped and hurled glass bottles and rocks at the red shirts until riot police forced them back with batons. The red shirts responded by throwing rocks and bottles back.

Local media said five people were detained, and later released.

One explosion hit outside the headquarters of Charoen Pokphand Group, Thailand's biggest agribusiness group. Another landed near the Dusit Thani Hotel. Others struck parts of the district.