Tunisians on Monday mark two years since the overthrow of ex-president Zine El Abidine Ben Ali amid a highly uncertain future beset by social and security tensions, a sluggish economy, and deadlock over a new constitution.

The hopes that accompanied the victorious first Arab Spring uprising have given way for many of its supporters to frustration at persistent poverty and hardship, despite Ben Ali fleeing to Saudi Arabia after 23 years in power.

"Contrary to what the government claims, the rate of unemployment has risen since the revolution and graduates represent more than a third of around one million job seekers," said Salem Ayari of the union for jobless graduates.

"Political tensions, nepotism and corruption have exacerbated an already critical economic situation."

A Western diplomat described the government, led by the moderate Islamist party Ennahda that triumphed in the 2011 parliamentary elections, as "running around in circles."

Popular frustration was starkly illustrated on December 17, when protesters heckled President Moncef Marzouki and pelted him with stones in Sidi Bouzid, the poor central town where the revolution erupted exactly two years earlier.

Strikes and protests have multiplied in the past year, often degenerating into violence, as in late November when around 300 people were hurt in running clashes between police and protesters in Siliana, a town southwest of Tunis.

This week, the southern border town of Ben Guerdane was rocked by violence, with protesters demanding investment and jobs torching a police station and customs office, and ransacking the premises of the ruling party.

On Friday, Ennahda chief Rached Ghannouchi issued a stark warning.

"We do not want Tunisia to become like Somalia, where revolution turned into chaos," he told supporters.

The government remains determined to forge ahead with the anniversary festivities, however, hoisting flags along the capital's streets and erecting tents to host cultural activities.

Representatives of both Libya and Egypt, two other Arab Spring states, will also attend Monday's ceremony.

-- Security Fears --

A "social pact" is also due to be signed on the day by trade unions, business leaders and the authorities.

Defending its record, the government points to the revolution's achievements such as freedom of expression and political pluralism, and a return to economic growth which went from negative in 2011 to 3.5 percent last year.

But beyond the generic social discontent and frequent confrontations between supporters of the government and its critics, the authorities are facing a separate, and more sinister threat, from the minority jihadist movement.

Last September, suspected Islamists attacked the US embassy in Tunis and a neighbouring American school in violence that left four people dead and dozens wounded.

Last month, security forces arrested 16 men suspected of belonging to a group linked to Al-Qaeda in the Islamic Maghreb in western Tunisia, after a deadly attack, and recovered weapons thought to have originated from Libya.

Security has been boosted ahead of the anniversary, with a security source saying there were concerns about possible attacks by a militant group whose members have been arrested in recent weeks.

"This group has weapons and represents a real danger for Tunisia" where a state of emergency has been in force since January 2011, the source said.

Politically, the situation is no less uncertain.

Ennahda has failed to reach a compromise with secularists in the interim parliament on the new constitution, which the Islamists had promised to have drafted by the end of October.

Legislative and presidential elections have been postponed to June and could be pushed back further, while negotiations on a possible cabinet reshuffle have dragged on since July, and absenteeism is hampering the work of lawmakers.

"We must agree to a serious and credible (electoral) calendar so that investors, for example, can know what to do," said Amira Yahyaoui, who heads an NGO that lobbies for transparency in parliament.