Gabon's opposition leader said security forces killed two people and hurt 19 in a raid against his headquarters Thursday, as violence erupted after President Ali Bongo was declared the winner of disputed polls.

Thousands of angry protesters poured onto the streets of Libreville late Wednesday, accusing the government of stealing the election after Bongo won a second term by a razor-thin margin over rival Jean Ping.

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

Police deployed near the parliament building fired tear gas canisters on Thursday to stop people gathering there.

As the country descended into chaos, the EU called for calm, former colonial power France urged "maximum restraint" and Amnesty International warned against "excessive force."

By mid-morning 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.

Two trucks packed with dozens of detainees who raised their fists and chanted the national anthem were seen being driven off for questioning, they said.

- Ping's whereabouts unknown -

Police chief Jean-Thierry Oye Zue told AFP that more than 200 people had been arrested across town for looting. He said looting was "currently continuing in the poorer districts." Telephone and internet communications were cut.

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

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.

- Helicopters used in raid -

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

"They attacked around 1:00 am (0000 GMT). 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 Ping, who was not himself at the party headquarters.

The president of the opposition National Union party, 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" and "looters and thugs" who had earlier set fire to the parliament building.

"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)," Ping told AFP.

A Red Cross worker who gave his name as Gildas said one of 15 people injured brought in by an army truck had died on Thursday.

- 'He cheated' -

The results of the presidential election, announced earlier Wednesday, handed Bongo a second term and extended his family's nearly five-decade-long rule.

The results -- which gave Bongo 49.8 percent to Ping's 48.23 percent (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.

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

Any appeal by 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 percent nationwide but soared to 99.93 percent in Haut-Ogooue, where Bongo won 95.5 percent of votes.

- 'Let's change together' -

"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.

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.

One third of Gabon's population lives in poverty, though the country boasts one of Africa's highest per capita incomes at $8,300 (7,400 euros) thanks to pumping 200,000 barrels of oil a day.

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.