A Thai car maker’s office party ended badly after employees started throwing glasses, bottles and even homemade grenades, injuring more than 30 people.

A brawl broke out as prizes were being awarded during the party thrown by AutoAlliance Thailand – a joint venture between Ford Motor Company and Japan’s Mazda Motor Corporation – for its 4,000 staff at a restaurant in Chonburi, southeast of Bangkok on Friday evening, although it was not clear what sparked it, reports the Bangkok Post.

“They were eating and many people were drunk,” police said. “Things got out of hand when some men threw bottles at each other”. Witnesses claimed they also heard gunshots.

More than 30 people were hurt and one was in critical condition, while the clash damaged a lot of property, Reuters reports.

“We have incidents like this every year,” a police officer said, who declined to be identified. AutoAlliance have yet to comment on the incident.