During the skirmishes, the second in three days, 20 students were injured, including six who were hospitalised, and 15 were detained for questioning and then released, a statement by the demonstration organisers said.

The protest was called by partisans of Aoun, a renegade Christian army general and head of a military government during Lebanon's civil war, who 15 years ago declared a "war of liberation" against the Syrian military presence in Lebanon.

The Lebanese government had banned protests and gatherings ahead of municipal elections scheduled in May.

Some 1500 students defied the ban, facing off for more than two hours a large contingent of anti-riot police and other law enforcement officers in downtown Beirut.

The clashes erupted when the students tried to march towards a government building and security forces turned water cannons on them and battered them with rifle butts, AFP correspondents said.

The students were demonstrating to mark the 15th anniversary of the "war of liberation" against the Syrian army Aoun had launched on March 14, 1989.

Syria once maintained more than 30,000 troops in Lebanon but has reduced its presence in its smaller neighbour in the past few years to around 20,000 soldiers.

On Wednesday, several Christian opposition students and soldiers were injured during clashes as police and troops attempted to prevent a demonstration from spilling beyondside a university campus, witnesses said.

A joint force of riot police and soldiers cordoned off the entrance to the Jesuit Universite Saint-Joseph where about 200 students from various Christian opposition groups were demonstrating, police said.

Friday's protesters also came from Saint-Joseph University, which was closed but surrounded by law enforcement agents.

But inside the campus students staged silent sit-ins while neighborhood residents took to the street in solidarity, AFP correspondents said.

In downtown Beirut the mood was more charged as protesters shouted anti-Syrian slogans.

They chanted "Syria Out" and "Freedom, Sovereignty and Independence".

They also denounced what they called "democracy imposed by guns" -- a reference to the authorities' systematic use of security forces to quell anti-Syrian protests.

"The policy of the regime, the corruption and the dependence on Syria are at the base of the emigration of youth," student leader Sami Gemayel said at Wednesday's protest.

Lebanon's Christian opposition is weak but is hoping to score points at the May municipal elections which is due to be followed in the fall by presidential elections and, within one year, by legislative polls.

On March 7 Aoun, who lives in exile in France, called for a "union of democratic forces, both Muslim and Christian," to beat supporters of Syria's dominance of the country in the May municipal vote.

"Corrupt politicians, in power with Syria's support, have divided up the various offices under cover of a fake conflict" between President Emile Lahoud and Prime Minister Rafiq Hariri, he told AFP.

"The forces for change must unite to defeat the candidates of the government and show that the Lebanese can live together and be governed without any foreign dominance," he said.

Aoun is sought by Lebanese authorities for statements he made before the US Congress -- when it was examining the possible adoption of sanctions against Syria -- which the authorities deemed harmful to Lebanese-Syrian ties.