The incident began after a speech by President Moncef Marzouki in the central Tunisian town, where celebrations are taking place on Monday to mark the anniversary of the revolution.

Mustapha Ben Jaafar, the parliamentary speaker, was about to address the crowd when the violence began.

Security forces swiftly evacuated the two men to the regional government headquarters, the AFP news agency reported.

The protesters invaded the square where the head of state had been addressing the crowd, shouting "the people want the fall of the government".

Growing anger

The police held back, after violent clashes over the past few months, which have often followed attempts to disperse protesters angry over the government's failure to improve living conditions in the poor region.

Clashes and strikes have multiplied across Tunisia in the run-up to the second anniversary of the start of Tunisia's revolution.

When the president took to the podium on Monday, many in the crowd of around 5,000 started shouting "Get out! Get out!" - one of the rallying cries of the revolution that toppled the regime of former dictator Zine El Abedine Ben Ali.

Marzouki promised economic progress within six months to the people of Sidi Bouzid, where poverty and unemployment were key factors behind the uprising that began there on December 17, 2010, after Muhammad Bouazizi a street vendor set himself on fire in protest at police harassment.

"I understand this legitimate anger. But the government has diagnosed the problem. In six months, a stable government will be in place and will provide the remedy to heal the country's problems," said the president, who was jeered by the crowd.

"For the first time, we have a government which is not stealing from the people," he said.

Marzouki had been heckled earlier in the morning, when he visited the grave of Bouazizi.