Liberal Party leader James Tien Pei-chun suggested yesterday that Chief Executive Leung Chun-ying should perhaps consider resigning after the massive turnout for the July 1 march.
He said the number that took to the streets was inexplicable given the current strong economy and the level of social stability in Hong Kong.
And he said the protest had dealt a severe blow to Leung's administration.
Tien pointed to his own resignation from the Executive Council days after the 2003 march, which saw 500,000 on the streets.
He said there was a combination of factors driving people to take part in the march that year, including discontent with a national security bill, the government's handling of the Sars crisis and a property market slump.
"Now, we have a jobless rate hitting a 16-year low and record high of tourist arrivals. Yet hundreds of thousands of people joined the march without specific negative factors such as those of 11 years ago," Tien said.
"The Leung administration is really having trouble governing.
"The government would be in bigger trouble if something really negative happens."
Embattled chief executive Tung Chee-hwa resigned in March 2005 citing health reasons. And Tien said: "Mr Tung resigned one-and-a-half years after the massive march in 2003.
"Maybe C.Y. Leung should consider, in a similar situation, whether he should resign … for the good of Hong Kong."
Tien supported former chief secretary Henry Tang Ying-yen's bid to challenge Leung in the 2012 chief executive election.
He said Leung, who has singled out implementing universal suffrage in 2017 as his priority during his term in office, had not done enough to help forge a consensus on political reform.
And he added: "If the government's reform proposal is vetoed by Legco next year, Leung should consider stepping down."
Tien said both Beijing and the people of Hong Kong should think about how to ensure a win-win situation for the policy of "one country, two systems".
He said: "The central government should appreciate Hongkongers' aspiration for democracy, while we should have a better understanding of Beijing's concerns, such as how to ensure national security in the process of electing the chief executive."