Hong Kong pro-democracy candidates won back only two of four seats in a crucial byelection in the semiautonomous Chinese region, final results showed Monday.

The opposition wasn't able to recapture all its territory, losing some to formidably resourced pro-Beijing rivals and falling short of the number needed to veto most bills in the city's semi-democratic Legislative Council.

The four seats were among six left empty when a group of lawmakers were expelled following a 2016 controversy over their oaths, which they used to defy China.

Little-known activist and neighbourhoodcouncillor Au Nok-hin won a key battleground after being enlisted at the last minute. He stepped in after election officials rejected the pro-democracy camp's marquee candidate, 21-year-old Agnes Chow, over her party's platform advocating "self-determination" for the former British colony, which she slammed as "political screening."

The disqualifications of lawmakers and candidates have raised fears among activists and international groups that Hong Kong's Beijing-backed government is taking an increasingly hard line on dissent.

"I won't say the result today is a victory," Au said after his results were announced. "I would say it's only a hollow victory, because we've paid a rather high price for it. The democracy camp has faced huge suppressions due to the political turmoil over these years."

Au had called the vote a referendum on democracy in Hong Kong, but the prospect of democratic development looks increasingly distant after China's rubber-stamp parliament voted Sunday to abolish presidential term limits, allowing President Xi Jinping to stay in power indefinitely.

Under the "one country, two systems" framework, Beijing promised to let Hong Kong maintain wide autonomy and civil liberties following its 1997 handover from Britain but fears are rising that China's communist leaders are backtracking.

Hong Kong's pro-democracy camp now holds 26 seats in the 70-seat Legislative Council. Two remaining empty seats will be decided later because they're the subject of ongoing legal action.

About 43 per cent of 2.1 million eligible voters cast ballots for three geographical seats, lower than the 58.3 per cent in 2016's citywide vote.

"It is significant that we didn't mobilize the people to vote. This is a serious challenge for 2020," when the next round of elections is scheduled, said Joseph Cheng, a retired political science professor, who added that residents "seem to be apathetic."

Edward Yiu, who was the only one of the disqualified lawmakers to compete again, narrowly lost to a pro-Beijing rival after a nail-biting recount. A third pro-democracy candidate, Gary Fan, won his constituency.

In a fourth race, architects and surveyors elected a pro-Beijing candidate over Dutch native Paul Zimmerman. About half of the council's seats are chosen by mainly pro-Beijing business and trade groups.

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AP journalists Josie Wong and Rafael Wober contributed to this report.