By Chris Hogg BBC, Hong Kong

The visit of Hong Kong's Chief Secretary, Donald Tsang, marks the start of a formal debate on constitutional reform in the territory.

Pro-democracy campaigners want Hong Kong's next leader to be elected, not appointed by Beijing.

Tung Chee-hwa is due to step down in 2007

But there are indications that the Chinese leadership will take a strong line on the issue.

This visit marks the start of official government-to-government dialogue about Hong Kong's political future.

Mr Tsang heads a taskforce created to look at whether universal suffrage should be introduced in the territory by 2007.

That is when Hong Kong's leader, Tung Chee-hwa, who was appointed by Beijing, is due to step down.

One of the demands of 500,000 protestors who took to Hong Kong's streets last summer was for the right to vote for his successor.

But there is confusion over whether Hong Kong's mini constitution, known as the Basic Law, would allow the election of the territory's leader in three years' time.

Mr Tsang is meeting senior officials and legal experts during two days of talks in Beijing to try to clarify the issue.

The Chinese leadership is expected to make clear its views on the pace of constitutional reform.

Senior figures on the mainland have hinted that Beijing has grave concerns about political developments since the 1997 handover.

Commentators suggest it is unlikely the Chinese leadership will take an understanding and sympathetic approach to the aspirations of Hong Kong's democracy activists, putting the two sides on a collision course in the run-up to elections to the territory's parliament, the legislative council, later this year.