Hundreds of people took to the streets of the Bahraini capital, Manama, to mark the fourth anniversary of an uprising that deeply divided the Gulf Arab state.

Police on Saturday fired tear gas and used rubber bullets at the mainly Shia Muslim demonstrators who were calling for political reforms by the ruling Sunni royal family.

They also called for the release of the main opposition leader Sheikh Ali Salman, who was arrested in December.

Protests comes after the opposition group February 14 Coalition, a cyber-youth group, urged demonstrations and strikes across the kingdom under the slogan "Strike of defiance".

But the public security chief, Major-General Tariq al-Hassan, warned that even calls to take part in the protests will be treated as a crime, and said people should "stay away from disruptive activities that might affect security or public order".

"Action would be taken against those who spread terror among citizens or residents, put the safety of others at risk or try to disrupt the nation's security and stability," Hassan said.

Najeeb Rajab, a prominent human rights campaigner in Manama who was recently released after serving two years in prison over charges related to his dissent, told Al Jazeera that hundreds of protesters have been arrested and many others were seriously wounded in a brutal police crackdown that began on Friday.

"Every five minutes protesters were shot at by police. We have a lot of people wounded now and we can not take them to hospitals because we are afraid they will be arrested," he said.

"Thousands of police have been deployed in the streets. And for the first time i have seen hundreds of armed people in civilian clothes attacking protesters."

On the other hand, protesters have also been accused of violence. According to the Associated Press news agency, in largely Shia areas on the capital's outskirts, young activists armed with rocks and gasoline bombs frequently blocked roads and fought police.

Uprising crushed

Bahrain's Saudi-backed Sunni authorities crushed protests led by its majority Shia shortly after they erupted on February 14, 2011, taking their cue from Arab Spring uprisings in the Middle East and North Africa.

Tensions are running high in the kingdom where a sectarian divide is deepening and there is a growing gap between the Sunni minority government and the Island’s Shia majority.

The opposition is demanding a "real" constitutional monarchy with an elected prime minister who is independent of the ruling royal family.

But the Al-Khalifa dynasty has refused to yield.

Shia opposition leader Sheikh Ali Salman is behind bars for allegedly trying to overthrow the regime.

His arrest on December 28, shortly after he was re-elected head of Bahrain's main opposition party Al-Wefaq, has sparked near-daily protests in Shia villages. Attacks targeting security forces have also increased.

"The movement has reached its four years with the situation only getting worse and deteriorating with citizens threatened by losing their nationalities any minute," Al-Wefaq said on Twitter.

In October, a court banned Al-Wefaq for three months for violating a law on associations.

Bahrain has revoked the citizenships of scores of activists over the past few years, drawing condemnation from rights groups.

Source: Al Jazeera and agencies