The protesters “have seriously undermined Hong Kong’s law and order and are pushing our city, the city that we all love and many of us helped to build, to the verge of a very dangerous situation," said Chief Executive Carrie Lam, who was appointed by a pro-Beijing committee.
"I dare say they are trying to destroy Hong Kong,” she added, as train travel and international flights were thrown into chaos.
Activists forced open doors to stop trains departing during the morning rush hour, stopping large parts of a network that millions of people use daily.
More than 160 flights at the city's airport, one of the world's busiest, were also cancelled.
The strike led to scuffles between angry commuters and demonstrators at crowded subway lines, while various other scenes circulated across social media.
One video showed a car smashing its way through a protester roadblock in the northern town of Yuen Long.
[22:20] Collected in Wong Tai Sin.JL Cool (@gromitcool) August 5, 2019
How many rounds of tear gas have Hong Kong police fired in Wong Tai Sin?#HongKongProtests #HongKong #HKstrike #85strike #PrayforHongKong
Source: Tony Tam https://t.co/QwvmlBA30s pic.twitter.com/X1hZjRGWps
Hong Kongers against China extraditions
The protests were triggered by opposition against a planned law that would have allowed extraditions to mainland China.
It quickly evolved into a wider movement for democratic reform, which has resulted in a fortnight of violence.
Hong Kong has rights and liberties under the terms of the 1997 handover deal with Britain, including independent judiciary and freedom of speech.
Protesters say many of those rights are being cut.
Simultaneous rallies were launched throughout the afternoon, with crowds filling up parks, public squares and malls.
Many shops across the city were shuttered, including fashion outlets in the central commercial district such as Topshop and Zara.
Dozens of activists have been charged with rioting, which carries a jail term of 10 years.
While some commuters were angered by the disruptions, others said they supported the action.
“As long as the government doesn’t respond then for sure the movement will escalate,” said a civil servant, who gave his surname as Leung.
Chinese authorities have largely left the government to deal with the crisis, although they have begun to issue their own warnings.
China's military last week described the unrest as “intolerable” and released a propaganda video showing troops quashing a protest in Hong Kong.
Police say that a flash mob is scheduled by protesters for Tuesday.