(Recasts, adds details)

By Paul Yeung

MACAU, May 2 (Reuters) - Lawmakers criticised Macau police on Wednesday for firing warning shots during a May Day march that erupted into violence, as authorities investigate if one of the bullets may have injured a passing motorcyclist.

Macau Security Police Chief Lei Siu-peng said on Wednesday the 50-year-old motorcyclist had been hit by a bullet and media reports said he had been struck while riding with his son some 300 metres from the protest.

Riot police confirmed they had fired five warning shots into the air during a rare protest march in the former Portuguese colony to demand better labour protection and an end to corruption.

Legislator Au Kam-san said police had no good reason to fire the warning shots as the protest was peaceful.

"Police can fire only if their safety is threatened but the march was largely peaceful. When police kept stopping the march and dragging away some protest leaders, people became angry," Au said. He will ask police to justify the gunshots to legislators.

Lei, the police chief, said 10 people were arrested and 21 police officers injured in the fracas involving 2,400 protesters.

FEARING STAMPEDE

"We are investigating the incident to see if there is any link between the bullet which wounded the man and the shooting ... but we have to emphasise that the gunfire was to prevent a stampede," he said.

Someone had fallen down during the fracas and, fearing a stampede, an officer fired the gunshots to warn the surging crowd against moving forward, Lei added.

Protesters and police had not agreed on the route of the march prior to May Day and tempers flared when police tried to stop protesters from entering a shopping belt that was packed with tourists, a Reuters photographer at the scene said.

Lei said police had received intelligence before May Day that "some people" may stir up trouble at the rally.

Macau has been a highly autonomous territory of China since 1999, after more than 400 years of Portuguese colonial rule, and unlike Hong Kong has little history of political protests.

On Tuesday, protesters carried banners and placards about workers' rights and corruption in the former Portuguese enclave. Some also demanded the resignation of Chief Executive Edmund Ho, Macau's Beijing-appointed leader.

The injured man was taken to Kiang Wu hospital, where he is conscious and recovering, after a bullet fragment was removed and handed over to police, Chan Tai-ip, a doctor from the hospital, told a news conference on Wednesday.

Au, the legislator, said the motorcyclist was injured around the time of the gunshots -- between 3.05 and 3.10 pm (0705-0710 gmt).

"If it was caused by police, it is unforgiveable and police must bear all responsibility ... but both the gun and the bullet are now in the hands of the police, so they can easily cover up," Au said.

Witnesses said police began beating people with batons and dragged away a number of them when protesters hurled water bottles and tried to break through police cordons. (Additional reporting by Donny Kwok and Tan Ee Lyn)

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