Gabon security forces have fired tear gas and arrested dozens of supporters of Andre Mba Obame, an opposition politician, during a protest in the capital Libreville, injuring several of them, according to one of his aides.

Obame denounced Thursday's crackdown as irresponsible. A former foreign minister and failed presidential candidate, he has taken refuge in UN offices in Libreville after declaring himself "president" two days ago.

Michel Ongoundou, defence minister in the "government" that Obame appointed, told the AFP news agency that soldiers and paramilitary gendarmes moved in on protesters at dawn outside the UN premises.

"There was tear gas fired. There have been people injured and about 30 arrests," he said.

An interior ministry spokesman told AFP that it was "possible that there have been arrests, but if these people are just supporters, they will not stay in prison".

Ongondou said that he had spent the night with 30 other opposition activists, including Obame, at the UN offices.

"I was on the terrace and I saw two lorries arrive, then a column of riot police. They immediately charged," he said.

The young opposition supporters started shouting, Ongondou said.

"Some had been injured and climbed the wall to take refuge here. Others were chased into the neighbouring districts."

Obame denounced the intervention by the security forces.

"It's total irresponsibility. You know that these kinds of acts often lead to incalculable consequences," he told AFP.

"There was really no need for that. It's similar behaviour to that which set off the riots at Port Gentil."

Rioting in the Gabonese city of Port Gentil killed at least three people in September 2009, after Ali Bongo, son of the country's late leader, was declared winner of a presidential election.

"All this is just throwing oil on the fire," Obame said.

"How do you think people are going to react? Each time it's the same."

Blaming Bongo for the intervention, he said: "He has to take responsibility, he is going to take responsibility."

Obame moved into the offices of the United Nations Development Programme demanding UN recognition.

Obame's claim on Tuesday that he was president, and his unveiling of a "government", prompted the 53-nation African Union to voice concern and call for the Gabon opposition to respect the law.

Hours after the announcement, the interior ministry announced that Obame's National Unity party was immediately dissolved and he and his supporters were stripped of their official duties and faced treason charges.