Activists hold yellow umbrellas during a gathering to mark the fourth anniversary of mass pro-democracy rallies, known as the Umbrella Movement, in Hong Kong on September 28. Photo: AFP

As room for opposition in semi-autonomous Hong Kong shrinks under an assertive China, three prominent democracy activists face trial on Monday over their involvement in huge rallies demanding political reform.

Rights groups have urged officials to drop what Amnesty International called the “chilling prosecution” of nine activists – the pioneering trio, lawmakers, student leaders and pro-democracy party campaigners.

All nine face “public nuisance” charges for their participation in 2014’s Umbrella Movement protests. The charges are based on colonial-era law and carry maximum prison terms of up to seven years.

Sociology professor Chan Kin-man, 59, law professor Benny Tai, 54, and Baptist minister Chu Yiu-ming, 74, founded the “Occupy Central” movement in 2013.

It called for the occupation of Hong Kong’s business district if the public was not given a fair vote for the city’s leader, who is appointed by a pro-Beijing committee.

The campaign was overtaken by a student movement that exploded in September 2014 when police fired teargas at protesters.

The Occupy trio urged people to join what became known as the Umbrella Movement because protesters used umbrellas to shield themselves from teargas and pepper spray.

The movement failed to secure the reforms they sought and since then activists have been prosecuted, with some jailed.

– With reporting from Agence France-Presse