Police clashed with thousands of anti-government protesters in Habib Bourguiba Avenue in Tunis on Monday, defying a ban on demonstrations in the area - a focal point of the revolt that ousted Zine al-Abidine Ben Ali more than a year ago.

At least 2,000 protesters came out to mark the Martyrs' Day holiday, which commemorates the 1938 suppression of pro-independence demonstrators by French colonial troops.

As they reached the interior ministry on Habib Bourguiba, they were met by police who fired tear gas to clear the crowd.

Police beat back groups of protesters with batons and chased stone-throwing youths down side streets in scenes reminiscent of the tactics used during Ben Ali's 23 years as president, when Tunisia was a police state and freedoms severely restricted.

The people want the fall of the regime, protesters chanted, echoing the demand that was coined in Tunisia during the 2011 revolution and sparked the Arab Spring uprisings.

The moderate Islamist Ennahda party, which won post-revolution elections in October, is under pressure from secular parties to improve economic conditions and not give religion too prominent a place in public life.

Monday's crackdown, the worst in the capital for months, is likely to prove a public relations nightmare for the Islamist-led coalition government, already reeling from an attack by activist hackers on the prime minister's emails.

Protesters on Monday likened Ennahda to the Trabelsi family of Ben Ali's wife Leila, widely blamed by Tunisians for the rampant corruption of the final years of his rule. The people are sick of the new Trabelsis, protesters chanted.

Tunisia has changed enormously since the revolution, with a democratic system now in place and ordinary people able to speak and demonstrate freely for the first time in memory.

But the interior ministry decided to ban rallies on Habib Bourguiba Avenue in late March after local hotels, restaurants and other businesses complained that repeated protests and counter-protests were snarling traffic and disrupting business.

The ban infuriated opponents of the government who chose Monday's public holiday to defy the police.

No fear, no terror, the street belongs to the people, the crowds chanted as they confronted police.

POLITICAL BACKLASH

Numbers at the rally quickly shrank, and groups of dozens of demonstrators faced off with police. Reuters journalists saw protesters fainting from tear gas and others hobbling away with bruises.

Social networking sites buzzed with rumours that some protesters suffered life-threatening head injuries at the hands of police. One Reuters witness heard police officers urging each other on to beat back protesers and journalists. It was not immediately possible to verify reports of serious casualties.

Tunisia's revolution ousted Ben Ali in January 2011. In October, Ennahda won more than 40 percent of seats in the constituent assembly in the country's first free elections.

From the outset, Ennahda has faced strong opposition from secular parties and Tunisia's powerful labour union, who fear it will impose conservative religious values on a country long known for its liberal and secular outlook.

Ennahda has promised not to ban alcohol or enforce wearing veils but has faced pressure from conservative Salafi Islamists pushing for a greater role for religion in public life.

The party, which leads the government in coalition with two secular groups, has tried to steer a middle course but the clashes are likely to cause a political backlash.

Protesters and opposition groups accused Ennahda of sending in masked thugs in plain clothes, who could be seen chasing protesters, to back police. An Ennahda official, speaking to the official TAP news agency, denied those claims and said its supporters were peacefully marking the day in another location.

The youth arm of the secular opposition party Ettajdid issued a statement on Facebook condemning the crackdown.

We call on the interior ministry to open an immediate inquiry to identify those responsible, it said.

We affirm our right to demonstrate, particularly on Habib Bourguiba, which is a symbol of the revolution, and our defence ... of the unconditional freedom of speech and protest.

(Additional reporting and writing by Lin Noueihed; Editing by Louise Ireland)