This is the harshest sentence to be given under the controversial national security law, which China says is necessary for the city's stability
Lai, who holds a British passport, was one of the loudest critics of Beijing, often wielding his pro-democracy paper, Apple Daily, as a tool of protest
Six former executives of the paper were also jailed on Monday - from six years and nine months to 10 years
Hailed a hero by the pro-democracy movement, Lai is seen as a traitor by Beijing
He has always denied the charges against him, saying he had advocated for what he believed were Hong Kong's values, such as rule of law and free speech
Lee was sentenced to 20 years, the harshest sentence to be given under Hong Kong's controversial national security law
The sentencing, which comes just a few months after Lai was found guilty of national security offences in December, was over in just around ten minutes
Other defendants, including six former Apple Daily executives and two activists who were also in court today, received sentences ranging from six years to 10 years
A number of human rights and media organisations have condemned Lai's sentence and called on the governments of China and Hong Kong to free him
Law's charges comprised two counts of conspiring to collude with foreign forces under the National Security Law, and one count of publishing seditious material on the now-defunct Apple Daily newspaper, which he founded, under a separate colonial-era law
Stay with us as we bring you more updates, reactions and analysis.
The head of Hong Kong police's National Security Department has defended Lai's 20-year sentence as "appropriate", according to Reuters.
Chief superintendent Steve Li was speaking after Lai's conviction in a Hong Kong court on Monday.
Lai, 78, received the harshest sentence to be given under Hong Kong's controversial national security law, which China says is necessary for the city's stability
Following the verdict, Li further said that claims of Lai's frail health were "exaggerated".
Lai's barrister had said he suffers from hypertension and diabetes, among other ailments, but the court said it was "not inclined" to hand Lai a lower sentence on account of his medical condition.