Lahore: An American man is expected to appear in a Pakistan court on Friday on double murder charges, police said, a day after he shot dead two men on a motorcycle in what he said was self-defence.

The man was described by the State Department in Washington as an American civilian working for the US Consulate in Lahore.

A third Pakistani was crushed to death by a consulate car that went to the scene to aid the man following the shooting in a busy street in the eastern city.

The incident threatens to inflame anti-American sentiment in Pakistan, already running high partly because of a covert US drone campaign in the northwest tribal areas that has provoked deadly revenge attacks by militants.

Police identified the gunman as Raymond Davis and said he described himself as a 'technical adviser' with the consulate.

"The post mortem of the dead men is underway, we are waiting for the report," police investigation officer Muneer Ahmed told AFP.

"Once we get the report and other documents we expect to produce the American before a magistrate for possible remand," he added.

The American national told police he shot the two motorcycle riders in self defence after they pulled a pistol on him in an attempted robbery, police officials said.

Two handguns were found close to the victims' bodies, officers said, but police chief Aslam Tarin said they so far appeared to have no previous criminal record.

Ahmed said another murder case had been registered against unknown foreigners in the second vehicle over the death of the third man.

Provincial law minister Rana Sanaullah said officials had asked the US Consulate to release the second vehicle and its driver to police.

The post mortem was delayed overnight because of protests by family members of the deceased, he said.

More than 100 people blocked the road after the incident, setting tyres on fire to protest against the killing and later moving their picket to the police station where the American's car was impounded.

In one sign of wider anger, Pakistan's right-leaning daily newspaper The Nation on Friday reported the incident under the headline: "'American Rambo' goes berserk in Lahore" and described Davis as an undercover US spy.

The US embassy in Islamabad has confirmed the man involved was a consular worker but said it was still trying to work out with the police what had happened.

There was confusion over whether the American was allowed to carry a weapon.

Police in Lahore said they were seeking legal advice on the matter, while a senior lawyer in Islamabad, Ali Zafar, told AFP that special permission was required for a foreigner to be armed.

"A foreigner can keep arms for his protection only with the special permission of the government. In this case police will have to ascertain whether the American had special permission or not," he said.

The United States said pm Thursday it wanted to avoid any anti-American backlash in Pakistan, a vital ally in the US-led 'war on terror'.

"We want to make sure that a tragedy like this does not affect the strategic partnership that we're building with Pakistan," State Department spokesman Philip Crowley told reporters.

"And we'll work as hard as we can to explain that to the Pakistani people."

Crowley gave few details of the event, but confirmed that an American civilian working for the US consulate in Lahore was 'involved in an incident', while declining to identify the individual.

"There's a Pakistan investigation. We will cooperate fully," he said.

Punjab minister Sanaullah said no American pressure would be allowed to influence the criminal case.

"We will not accept any pressure in this case. The guilty will be punished and the innocent will get justice," he told reporters.

"There was no reason for him (the American) to go into such a congested area and we are also investigating this."