Masked Hong Kong riot police stopped and searched people arriving on Saturday for a march billed as a global "emergency call" for autonomy for a city that was guaranteed its freedoms when it returned from British to Chinese rule in 1997.
The crowd of more than 3,000, many wearing black and in now-banned face masks, some carrying British and colonial flags, was building fast in Victoria Park, next to the Causeway Bay shopping district, under a hot autumn sun.
Pro-democracy activist Joshua Wong urged 100,000 people to take part on the 22nd straight weekend of protest.
Anti-riot Police Tactical Unit vans were already in waiting on the narrow side-streets around the Central business district to the west, the marchers' destination later in the afternoon.
Protesters have taken to the streets for five months of sometimes violent unrest, angry at perceived Chinese meddling with Hong Kong's freedoms, including its legal system and politics. China denies the charge.
Activists have attacked police with petrol bombs, set street fires and trashed government buildings and businesses seen as pro-Beijing.
One policeman was slashed in the neck with a knife last month.
Police have responded with tear gas, pepper spray, water cannon, rubber bullets and occasional live rounds. Several people have been wounded.
Saturday's rally was not given official police permission, as is required, but that has not stopped people gathering in the past. Many were wearing face masks, banned under a resuscitated colonial-era emergency law which has been widely ignored.
Around 100 district council election candidates were meeting small gatherings of people. Under the law, they can meet with up to 50 people per candidate, which they say gives them cover to assemble 5,000 people in the park legally.
Wong was disqualified on Tuesday from standing in the upcoming district elections, a move he said was "clearly politically driven".
"If more and more people, not only a few thousand, but if more than 100,000 Hong Kongers take to the streets tomorrow, it can let the world know how Hong Kong people fight for a free election," he told reporters on Friday.
The bespectacled Wong was a leader of the student-led pro-democracy street protests of 2014 but has not been in the forefront of the current unrest.
Government data on Thursday confirmed that Hong Kong slid into recession in the third quarter for the first time since the global financial crisis of 2008. Retail sales fell 18.3% in value in September from a year earlier, an eighth consecutive month of decline.
Hong Kong is a semi-autonomous "special administrative region" of China according to the "one country, two systems" formula under which it returned to Chinese rule.