By nightfall on Wednesday police and protesters were again clashing on the streets, with riot officers shooting tear gas almost immediately, as their response to demonstrators toughens.
Flights resumed at Hong Kong airport, which is one of the world's busiest, after two days of disruptions.
Thousands of protesters have occupied the airport for days, forcing the cancellation of hundreds of departures on Monday and Tuesday.
Ten weeks of increasingly violent confrontation between police and protesters have plunged the city into its worst crisis since it reverted from British to Chinese rule in 1997.
China's Hong Kong and Macau Affairs Office in Beijing called the behaviour at the airport no different to terrorism and said it must be severely punished.
A group of a few dozen demonstrators held up an apologetic banner in the airport arrivals hall.
"We're deeply sorry about what happened yesterday," it read.
"We were desperate and we made imperfect decisions. Please accept our apologies."
In chaotic scenes that would once have been unthinkable for Hong Kong, a peaceful sit-in at the airport turned violent late on Tuesday as protesters confronted and held a man they believed was an undercover Chinese agent.
Busloads of riot police arrived in response, clashing with furious demonstrators before withdrawing once the man was removed, and leaving the terminal briefly in control of activists who then detained a Chinese reporter for a short time.
The protests have hit the city's faltering economy.
"We promise to reflect and to improve," protesters said in one message distributed on social media app Telegram.
"Sorry we were too reckless ... we are only afraid of losing your support to the whole movement due to our mistake, and that you give up on fighting."
They also showed little sign of relenting in their protests, which began in opposition to a now-suspended bill that would have allowed the extradition of suspects for trial in mainland China, but have swelled into wider calls for democracy.
Hundreds attended a demonstration in the residential area of Sham Shui Po, where police arrived and quickly used tear gas after protesters pointed lasers at the police station.
China used its strongest language yet after Tuesday's incidents, when the protesters seized a reporter from China's Global Times newspaper, a nationalistic tabloid run by the ruling Communist Party's official People's Daily, and harassed the man they believed to be a mainland agent.
In addition to Beijing's condemnation, the People's Daily called for "using the sword of the law" to restore order, and mainland social media users lauded the detained reporter as a hero.
German Foreign Minister Heiko Maas said during a visit to Toronto on Wednesday that all sides involved must ensure the situation does not escalate.