Manama - Armoured vehicles patrolled Bahrain's capital in a security clampdown on Tuesday to deter protesters after overnight clashes outside Manama on the first anniversary of a pro-democracy uprising crushed by the Gulf Arab kingdom.

Youths flung volleys of petrol bombs at police cars during pre-dawn skirmishes, prompting authorities to impose a massive police presence in Shi'ite Muslim villages ringing Manama, with helicopters buzzing overhead.

The re-emergence of armoured personnel carriers for the first time since martial law was lifted last June underlined the concerns of the Sunni Muslim-led monarchy about a new explosion of civil unrest by Bahrain's disgruntled Shi'ite majority.

Shi'ite protests have revived in the lead-up to the anniversary of the February 14, 2011 uprising, when mainly Shi'ite protesters occupied Manama's Pearl Roundabout for a month before security forces - with the help of Saudi troops - broke up the movement inspired by revolts in Egypt and Tunisia.

The growing anger among Shi'ites, who complain that they are treated as second-class citizens and shut out of plum jobs with limited access to good housing, is a complicated challenge for a Sunni ruling elite in power for over 200 years.

Bahrain is an important ally of the West in its hosting of the U.S. Fifth Fleet to counter Shi'ite Iran across the Gulf.

The United States has put on hold a key arms deal until it sees “more progress” by the Bahraini government on reforms.

The roundabout, which had a giant concrete edifice featuring a pearl that was later taken down, was shut to traffic and remains that way, renamed as al-Farouq Junction, and under tight guard. That security was beefed up in recent days as opposition activists sought to reclaim the symbolically rich space.

On the eve of the anniversary, hundreds of protesters broke away from an authorised opposition party rally to march down the main highway into Manama, bound for the roundabout, before police stopped them with tear gas and rubber bullet pellets.

Street battles ensued with youths throwing petrol bombs, rocks and iron bars. They chanted in favour of Hassan Mushaimaa, a jailed Shi'ite leader who called for a republic last year.

The junction remains enclosed by barbed wire on most sides and security guards have set up an encampment nearby.

King Hamad bin Isa Al Khalifa told the nation in an address on state television on the anniversary eve that he remained committed to reforms launched a decade ago, a process the opposition has dismissed as cosmetic.

“(This) marked the launch of a development and modernization process, which is still moving forward to meet the aspirations of our loyal people in all areas,” said the king, whose family has run the Gulf island state for over 200 years.

He said he had pardoned 291 prisoners, but they did not include those arrested during last year's revolt. The opposition are demanding the release of 14 leading figures who were jailed by a military court for allegedly trying to stage a coup.

Young men justified this week's disturbances by saying they were in constant conflict with police who treat them harshly. “This is just one way of expressing our protest,” said one, who declined to give his name out of concern for his own safety.

He said they were ignoring calls by Sheikh Ali Salman, head of the leading Shi'ite opposition party Wefaq, not to throw petrol bombs. Analysts say Wefaq, which supports the monarchy, fears losing support to figures like Mushaimaa.

“We respect the opposition but everyone has to choose their own path. Ali Salman doesn't really know the situation we live in,” he said.

Feb. 14 is not only auspicious for last year's uprising. It is the date of a 2001 referendum on a national charter on reforms King Hamad introduced to end a 1990s uprising.

Opposition parties say the constitution promulgated a year later was a disappointment because it neutralised the powers of an elected assembly with an upper house of royal appointees.

Wefaq and other opposition parties, including the secular Waad led by jailed Sunni political Ibrahim Sharif, want constitutional changes that would give the elected chamber of parliament the authority to form governments.

After last year's unrest, the government granted parliament extra powers of scrutiny over ministers and budgets - but has not budged on the more far-reaching opposition demands.

Bahraini authorities have hired U.S. and British police chiefs to help reform policing after revelations about torture and deaths of detainees during last year's crackdown.

One of them, former Miami police chief John Timoney, told Reuters this week the interior ministry was serious about reform and would hire Shi'ites in a new recruitment and training drive, but he said youth violence was posing obstacles.

Opposition parties and youths say they have noticed no improvement in police behaviour and accuse police of using harsh tactics for political reasons - to suppress dissent in Shi'ite villages that could form a critical mass of protesters again.

Despite the government's professed reform efforts, it has not been enough to convince U.S. lawmakers to unfreeze a planned $53 million arms sale to Bahrain.

Bahrain says it needs the hardware, including armoured Humvee vehicles and missiles, to defend itself from non-Arab Shi'ite giant Iran, which it accuses of fomenting the revolt to turn Bahrain into an Islamic republic. Iran denies this.

State Department spokeswoman Victoria Nuland said on Monday that the United States will not go ahead with the deal until Bahrain makes more headway in implementing reforms.

More progress was needed, Nuland said, including reinstatement of workers unfairly dismissed from their jobs and resolution of ongoing court cases related to political expression.

“More remains to be done on that. Assistance is still on pause ... We're not going to go forward until we see more progress,” Nuland told reporters.

The State Department last month authorised $1 million of equipment to Bahrain to support U.S. operations in the region, including the Bahrain-based Fifth Fleet. - Reuters