BRUSSELS (AFP) - The European Union on Thursday (Sept 1) for all sides in Gabon to reject violence as the country descended into chaos after President Ali Bongo was declared winner of disputed polls.

"The official announcement of results has plunged Gabon into a deep crisis," European Union foreign policy chief Federica Mogherini said in a statement. "It is important for all parties to reject violence and call for calm."

Thousands of angry protesters poured onto the streets of Libreville accusing the government of stealing the election after Mr Bongo won a second term by a razor-thin margin over rival Jean Ping, who said security forces killed two people on Thursday.

Ms Mogherini said "any challenge must be made with peaceful means in order to avoid any conflagration in the country. Security forces must react responsibly".

"Trust in the election results can only be restored through a transparent verification, polling station by polling station."

She added that the EU is in contact with all its African and other partners in order to promote a peaceful resolution to the crisis.

More than 200 people have been arrested for looting in capital Libreville, where violence broke out following the result, the police chief said on Thursday.

"We have arrested more than 200 looters since last night," national police chief Jean-Thierry Oye Zue told AFP. "Looting is currently continuing in the poorer districts."

Gunfire crackled across the city and plumes of smoke billowed from the torched Parliament building as protesters clashed with heavily armed security forces.

By mid-morning on Thursday, security forces had sealed off the city centre, which was calm and otherwise deserted, and were making arrests around the opposition headquarters, AFP journalists said.

It was not immediately clear where Mr Ping - a veteran diplomat and former top African Union official who had earlier declared himself the poll winner - had taken refuge.

Scenes of pillaging were reported from outlying districts and telephone and internet communications were cut.

The Parliament building's facade was blackened by fire and its windows were smashed. Protesters had torn down its huge main gate and torched a sentry box at the entrance.

On the city's main artery, the Boulevard Triomphal - the location of numerous government institutions and foreign embassies - burnt-out buildings and cars could be seen, while makeshift barricades were still smouldering.

Security forces had surrounded the opposition headquarters overnight and stormed the building, killing two and injuring more than a dozen there, Mr Ping told AFP.

"They attacked around 1am. It is the Republican Guard. They were bombarding with helicopters and then they attacked on the ground. There are 19 people injured, some of them very seriously," said Mr Ping, who was not himself at the party headquarters.

The president of the opposition National Union party, Mr Zacharie Myboto, who was inside the besieged building, said security forces were hurling tear gas canisters and had opened fire. "For nearly an hour the building has been surrounded. They want to enter the building... it is extremely violent," he said shortly after the siege began.

A government spokesman said the operation was to catch "criminals" who had earlier set fire to the Parliament building. "Armed people who set fire to the parliament had gathered at Jean Ping's headquarters along with hundreds of looters and thugs... they were not political protesters but criminals," said Mr Alain-Claude Bilie-By-Nze.

"We have said that the people of Gabon are in danger. They (the international community) should come and help us against the clan (of Bongo)," Mr Ping told AFP.

The results of the presidential election, announced earlier on Wednesday, handed Mr Bongo a second term and extended his family's nearly five-decade-long rule. However, the results - which gave Mr Bongo 49.8 per cent to Mr Ping's 48.23 per cent (a gap of less than 6,000 votes) - remain "provisional" until approved by the constitutional court.

The opposition described the election as fraudulent and called for results from each of Gabon's polling stations to be made public to ensure the credibility of the overall outcome - a demand echoed by the United States and European Union.

"This will help give the people of Gabon - as well as the international community - confidence the announced vote tallies are accurate," said US State Department spokesman John Kirby.

Mr Ping told France's Europe 1 radio early on Thursday: "Everyone knows that he (Bongo) cheated. Ask in Europe. Everyone knows."

EU observers said the vote was "managed in a way that lacked transparency" and opposition delegates in the electoral commission have already vowed to fight for a recount.

Any appeal by Mr Ping would likely focus on disputed results in Haut-Ogooue province, the heartland of Bongo's Teke ethnic group.

In Saturday's vote, turnout was 59.46 per cent nationwide but soared to 99.93 per cent in Haut-Ogooue, where Mr Bongo won 95.5 per cent of votes. "It's going to be difficult to get people to accept these results," one member of the electoral commission told AFP, asking not to be named.

"We've never seen results like these, even during the father's time," he added.

Mr Bongo took power in 2009 in a violence-marred election that followed the death of his father Omar Bongo, who had governed the oil-rich former French colony for 41 years.

UN Secretary General Ban Ki Moon called for calm and voiced concern over the clashes and arson, urging political leaders "to address their differences peacefully and to address any disputes they may have through existing constitutional and legal channels".

Former colonial power France said it was "extremely concerned" by the situation and urged "maximum restraint" on all sides.

One third of Gabon's population lives in poverty, though the country boasts one of Africa's highest per capita incomes at US$8,300 thanks to pumping 200,000 barrels of oil a day.

Mr Bongo, 57, campaigned under the slogan "Let's change together", playing up the roads and hospitals built during his first term and stressing the need to break with the bad old days of disappearing public funds and suspect management of oil revenues.