BEIJING, May 13 (Reuters) - Workers replaced the giant portrait of late Chinese Chairman Mao Zedong that hangs above the gate to the Forbidden City with a new replica on Sunday after the former was defaced by a vandal, Xinhua reported.

Police were interrogating Gu Haiou, a 35-year-old from the far-northwest region of Xinjiang after he hurled a burning object at the portrait Saturday afternoon, the report said.

It left a slight burn mark in the lower left part of the portrait, Xinhua quoted Beijing Public Security Bureau as saying.

Gu was a patient in a mental hospital in the city of Urumqi last year and arrived in Beijing on Saturday, the report said.

The damaged portrait, six meters high, five meters wide and weighing about two tonnes, is a reproduction of the fourth edition of Mao's portrait that has hung at the historic spot since Mao founded new China in 1949, Xinhua said.

The site on Tiananmen square is considered especially sensitive since it was the location of the 1989 pro-democracy demonstrations that were eventually crushed by the military.

The area is usually swarming with plainclothes policemen who quickly stamp out any signs of dissent.

China's leadership brooks no challenges to its authority, and defacing Mao's portrait could meet stiff punishment.

Chinese journalist Yu Dongyue was jailed for more than 16 years for hurling eggshells filled with red paint at the Mao portrait at the height of the 1989 demonstrations.

He was diagnosed as mentally ill by the time he was released in February 2006.

Despite leading the country in a series of violent political movements that led to millions being purged or killed and the economy moribund, Mao is still revered by many in China and is seen as a symbol of its strength and unity.

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