The rare violence raised concern about a crackdown ahead of more planned protests after Friday prayers in different cities throughout the oil-rich kingdom. The pro-Western monarchy is concerned protests could open footholds for Shiite powerhouse Iran and has accused foreigners of stoking the protests, which are officially forbidden.

Despite the ban and a warning that security forces will act against them, protesters demanding the release of political prisoners took to the streets for a second day in the eastern city of Qatif. Several hundred protesters, some wearing face masks to avoid being identified, marched after dark asking for "Freedom for prisoners."

Police, who were lined up opposite the protesters, fired percussion bombs, followed by gunfire, causing the crowd to scatter, a witness said, speaking on condition of anonymity for fear of government retaliation.

The witness said at least one protester was injured and lifted by others to a car for treatment. It was not clear how the protester was injured.

Scores of protesters in Qatif had also marched in the city streets Wednesday night.

Mainly Sunni Saudi Arabia has struggled to stay ahead of the unrest that has led to the ouster of the Egyptian and Tunisian leaders in recent months.

Last month, the ultra-conservative government announced an unprecedented economic package worth an estimated $36 billion that will give Saudis interest-free home loans, unemployment assistance and debt forgiveness. It also has reiterated that demonstrations are forbidden in the kingdom because they contradict Islamic laws and society's values and said security forces were authorized to act against anyone violating the ban.

So far the demonstrations have been small and concentrated in the east among Shiites demanding the release of detainees. But activists emboldened by other uprisings have set up Facebook groups calling for protests in the capital, Riyadh, on Friday to demand democratic reforms. One such group garnered more than 30,000 supporters.

The spread of calls for protests, particularly outside the generally tense eastern Saudi Arabia, home to the country's Shiite minority, has prompted government officials to issue strong warnings that it will act against activists taking to the streets.

Amnesty International called on Saudi authorities to reverse the ban on peaceful protests in the kingdom.

Philip Luther, a spokesman for the international rights group, said authorities should address the need for major human rights reforms and heed the growing calls for change instead of trying to intimidate protesters.

"Reports that the Saudi authorities plan to deploy troops to police upcoming demonstrations are very worrying," he said.

The Interior Ministry has banned demonstrations, saying they contradict Islamic laws and society's values and adding that some people have tried to go around the law to "achieve illegitimate aims."

"Reform cannot be achieved through protests ... The best way to achieve demands is through national dialogue," Saudi Foreign Minister Prince Saudi al-Faisal said Wednesday.