BEIJING, Nov 20, 2008 (AFP) - Police have arrested 30 people for involvment in a two-day violent protest in northwest China that had to be broken up with tear gas after 74 people were injured.

As calm returned to Longnan in Gansu province, authorities said they had questioned 110 people over the riots during which people threw stones, bricks and bottles to attack police, according to a local government statement.

The statement said 30 people had been detained -- seven of whom have been bailed pending further investigation -- and 80 people had signed confessions.

Damages are estimated at more than five million yuan (731,000 U.S. dollars), but that number could go higher, he said.

The government said the protest was triggered on Monday by about 30 people whose houses had been demolished to make way for a new government building, in an apparent typical "land grab" case that often leads to protests in China.

The protest swelled to about 2,000 people on Tuesday but had been stamped out by the evening amid a strong security presence and a lockdown on traffic.

Among the 74 people injured -- armed and regular policemen, and three journalists -- 11 were in hospital, according to the statement.

"So far as I know, 10 police officers and one journalist, who were injured by stones, bricks and wine bottles, are being treated in the hospital," said Li.

Demonstrations are common in China, often fuelled by government-backed land grabs or other types of abuse of power.

But as the global economic slowdown hits China and thousands of workers are laid off, the Communist Party is becoming increasingly concerned that any protest might escalate into broader unrest and form a challenge to its rule.