Hong Kong authorities have disqualified 12 pro-democracy candidates from upcoming elections, deepening political tensions in the Chinese territory.

Opposition legislators had hoped to obtain a majority in the Legislative Council (LegCo) in September's poll after Beijing's imposition of a highly controversial national security law.

Among those barred are high-profile activists Joshua Wong and Lester Shum.

The government said the candidates were not fit to run for office.

It said they could not be considered to be abiding by the constitutional duty required of lawmakers if they:

advocated for, or promoted, Hong Kong's independence
solicited intervention by foreign governments in Hong Kong's affairs
expressed "an objection in principle" to the imposition of the national security law by central authorities in Beijing
expressed "an intention to exercise the functions of a LegCo Member by indiscriminately voting down" any legislative proposals introduced by the Hong Kong government, "so as to force the government to accede to certain political demands"

In its statement announcing the disqualifications, the government said the decision was taken in line with Hong Kong's mini-constitution - the Basic Law.

"There is no question of any political censorship, restriction of the freedom of speech or deprivation of the right to stand for elections as alleged by some members of the community," it said, adding that more disqualifications could not be ruled out.

Joshua Wong, who rose to prominence as a teenage activist during protests in 2014, said the decision showed "a total disregard for the will of Hongkongers" and "tramples upon the city's last pillar of vanishing autonomy".

The new national security law has been highly controversial in Hong Kong, a former British colony which is now part of China but was given unique freedoms in an agreement before the transfer of sovereignty.

The law was widely condemned by Western governments, but China says it is necessary to restore stability in the territory, which was hit by months of pro-democracy protests last year which often turned violent.

The opposition candidates disqualified on Thursday include four incumbent lawmakers, four district councillors - including Mr Shum - and activists Ventus Lau Wing-hong, Gwyneth Ho Kwai-lam and Alvin Cheng Kam-mun, in addition to Mr Wong.

The Civic Party, one of the city's pro-democracy parties that had members among those barred, said the disqualifications "exploited the right of Hong Kong people to vote", Reuters news agency reports.

Its four disqualified members were Alvin Yeung, Dennis Kwok, Kwok Ka-ki and Cheng Tat-hung.