A brawl broke out as prizes were being awarded at a Thai carmaker's office party and more than 30 people were injured in the melee in which glasses, bottles and even homemade grenades were thrown, police and media said on Saturday.

AutoAlliance Thailand, a joint venture between Ford Motor Company and Japan's Mazda Motor Corporation, threw the party for its 4,000 staff on Friday evening at a restaurant in Chonburi, 100 km (63 miles) southeast of Bangkok.

"They were eating and many people were drunk," said a police officer who went to the restaurant to help restore order. "Things got out of hand when some men threw bottles at each other."

The Bangkok Post newspaper said the brawl started in front of a stage as prizes were being handed out. It was not clear what sparked it.

"The chaos quickly spread as glasses and other missiles were thrown with periodic explosions and gunshots also heard, according to some witnesses," the newspaper said. "A lot of property was also damaged or destroyed."

More than 30 people were hurt and one was in critical condition, the newspaper said. AutoAlliance spokesmen were not available for comment.

"We have incidents like this every year," said the police officer, who declined to be identified.