BEIJING (AFP) - Beijing's Tiananmen Square was evacuated by the police on Monday as pictures posted online showed a vehicle in flames in front of the Forbidden City.

"A motor vehicle went into the crowd in front of the Tiananmen rostrum on Monday noon," the official Xinhua news agency reported, as a security operation went into effect on the square, the site of huge pro-democracy protests in 1989 brutally crushed by the authorities.

Pictures posted on Chinese social media sites showed a plume of black smoke rising in front of the portrait of Mao Zedong hanging on the towering wall of the former imperial palace, police vehicles, and crowds looking on.

Several were deleted within minutes, streets leading to the square were blocked off, and two AFP reporters were detained close to the site.

Beijing transport authorities said on a verified social media account that a subway station next to the square had been closed at the request of police.

One 58-year-old Italian tourist said he had been visiting the Forbidden City when officers came in around noon and told everyone to exit.

Tiananmen Square is the symbolic centre of the Chinese state and is generally kept under tight security, with both uniformed and plain-clothes personnel deployed, many of them equipped with fire extinguishers.