Mr Tung flew to Beijing on Friday

Although Mr Tung's resignation ended days of speculation about his future, it opened up new questions about his successor and their length of term.

Mr Tung's deputy, Donald Tsang, is set to become acting chief executive until a permanent replacement is chosen.

Mr Tung flew to Beijing on Friday, where his resignation is set to be formally accepted.

His departure marks the first leadership change since Hong Kong returned to Chinese rule in 1997.

Under the Basic Law, Hong Kong's mini-constitution, the Beijing-appointed 800-strong committee which chooses Hong Kong's leader must hold an election within six months.

HAVE YOUR SAY The exit of Mr Tung closes a turbulent chapter in the post-colonial history of Hong Kong Alan Chan, Hong Kong Send your reaction Mixed press for Tung

The BBC's Chris Hogg in Hong Kong says Mr Tsang is the front-runner to fill the post on a permanent basis.

However, it is not yet clear whether Mr Tung's successor will merely serve out the remaining two years of his term or have a five-year mandate of his own.

Mr Tsang made no comment to reporters on Friday as he walked to his office.

Pro-democracy campaigners called for Beijing to use the hiatus to allow the Hong Kong people to start choosing their rulers themselves.

"We've seen a lot of interference from Beijing from the start. Hong Kong people do not have the high degree of autonomy they were promised," said Jackie Hung, spokeswoman for the Civil Human Rights Front (CHRF) group.

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Activists who have organised two huge pro-democracy protests in Hong Kong in the last two years, said they were planning another demonstration on Friday night.

Hong Kong's newspapers were sceptical on Friday after Mr Tung's purported reasons for leaving.

He told a news conference that he was stepping down after eight years because of declining health, but critics have noted that he was not specific about his alleged ailments.

Many observers in Hong Kong believe that he was in fact sacked by the Chinese leadership.

He had faced growing unpopularity over failed plans for an anti-subversion bill and his continued support for Beijing in limiting democratic reforms.

Rumours of Mr Tung's impending departure began circulating last Monday after details of the 67-year-old former shipping magnate's new job emerged - vice-chairman of the Chinese People's Consultative Conference.

Many saw this as a way of easing him out of office in Hong Kong.