Tens of thousands of people marched through Hong Kong on Sunday to protest the slow pace of democratic reform in this former British colony, but the territory's Beijing-backed leader again rejected demands to set a timetable for achieving universal suffrage.

Police and protest organizers gave sharply conflicting estimates of the size of the march, but the demonstration appeared to be the largest in Hong Kong since the huge anti-government rallies in 2003 and 2004 that drew crowds of more than 500,000 people into the streets demanding the right to elect the city's leaders.

Organizers said more than a quarter-million people participated in Sunday's protest, which was intended to pressure the Chinese government and the man it picked to run Hong Kong, Chief Executive Donald Tsang, to modify a proposed package of limited political reforms. Police said about 63,000 participated in the protest.

Tsang's plan would add seats to the legislature and expand the committee that China uses to name the chief executive, but the pro-democracy opposition has condemned it as inadequate and vowed to block the proposal when it comes to a vote later this month.

Speaking at a news conference Sunday night, Tsang struck a conciliatory tone and said he would see what he could "do to perfect the package." But he said the march had not persuaded him to make significant changes, and he rejected the opposition's main demand for a timetable for introducing general elections.

Instead, Tsang said, he would work on producing a timetable after his plan was enacted. "I am 60 years of age. I certainly want to see universal suffrage taking place in Hong Kong in my time," Tsang said. "My feeling and my wish is the same as most other people participating in the rally today."

China has ruled out direct elections in 2007 and 2008 to choose the territory's next leader and all its legislators, and it has refused to say when it will fulfill a long-standing promise to allow universal suffrage in Hong Kong.

Pro-democracy lawmakers called on Tsang to overhaul his proposal in light of the size of the protest. "Any responsible government should make a positive response to this march," an independent legislator, Ronny Tong, told reporters.

A small group of protesters began a sit-in outside government headquarters after Tsang's news conference, and march organizers said they were considering staging another large demonstration next weekend if Tsang refused to offer concessions.

In a development that could give a boost to the democracy movement, one of Hong Kong's most popular politicians -- a career civil servant who has not participated in similar demonstrations in the past -- joined the march Sunday.

"I feel that there are moments in one's life when one has to stand up and be counted, and for me, I believe this is one of these moments," said Anson Chan, who stepped down as the territory's number two official in 2001 and is often described as "Hong Kong's conscience."

Chan said she decided to march in the protest after a hastily convened meeting that Chinese officials held with pro-democracy legislators and community leaders this past week. At the meeting, Chinese officials declared that it would be unlawful to set a timetable for achieving universal suffrage.

On Sunday, Chan's voice was quickly drowned out by applause and cheers when she was spotted in the crowd. Analysts consider Chan a potential candidate in any future election to lead Hong Kong and say the government might be forced to offer a compromise if the public rallied around her.

Stanley Ho, a casino tycoon in nearby Macau who is a power broker in the region and serves as an adviser to the Chinese government, dismissed the protest as "insignificant" and warned the public against angering Beijing. "If 500,000 people came out, the government might need to do something," he told reporters.

Bernard Chan, a member of Tsang's council of advisers, also said the government would not offer any concessions, adding that only the people of Hong Kong would suffer if the limited reforms that Tsang has proposed were rejected.

The reforms would double the size of the 800-member Election Committee that chooses the chief executive and add 10 seats to the legislature. Analysts say that would introduce a measure of greater representation to the political system but keep the government firmly under Beijing's control.

Tsang has campaigned aggressively for the proposal, and he delivered an unprecedented televised address on Wednesday to answer his critics, arguing that the reforms were the best that Hong Kong could currently win from nervous Chinese leaders.

But many in the crowd of demonstrators -- who wore black T-shirts and waved banners saying "We want direct elections!" -- demanded faster change.

"We are not anti-China. We are here to fight for the rights we should have as citizens," said Tang Bok-man, 74. "I probably cannot see full democracy in Hong Kong in my life, but I hope my children and grandchildren can enjoy it."

Correspondent Philip P. Pan in Beijing contributed to this report.

Anson Chan, center, the former second-ranking official in Hong Kong, arrives to "stand up and be counted" with pro-democracy protesters.