HONG KONG (AFP) - A Hong Kong student who helped found a network of volunteer medics to aid the city's pro-democracy protests has been arrested on the mainland, fellow students briefed by his university have told AFP.

Classmates say Kwok Chun Fung has been detained on charges of "soliciting prostitutes" in Guangzhou, the capital of the province that borders Hong Kong.

The same allegation was brought against Mr Simon Cheng, a Hong Kong employee of the British consulate who was detained during a trip to the mainland in August last year.

After his release, Mr Cheng said he had been forced to confess before fleeing overseas.

Kwok, who is in his early 20s and studies in Guangzhou, co-founded a volunteer group that helped wounded protesters and those affected by police tear gas during the seven months of unrest that has engulfed Hong Kong.

His disappearance was first made public on Jan 10 by a co-founder of the group and his mother, who said he had gone missing in the city two days earlier.

The fellow volunteer - who uses the pseudonym "Small Cow" - said he was talking to Kwok when the line suddenly went dead.

"We were chatting on the phone and he said he had to get up and open the door. Then I heard him walking, opening a door and the phone was dropped, the call cut," he told reporters.

This week, Kwok's university - the Guangzhou University of Chinese Medicine - briefed non-mainland students on his disappearance, according to two students who attended the meetings.

University officials told them Kwok had been arrested late at night on Jan 8 for "soliciting prostitutes" in a hotel, an offence punishable by up to 15 days in administrative detention.

"During the briefing, the teacher told us to cherish our chance to study here as we will be graduating in half a year," one of the two students told AFP, requesting anonymity.

A second student who attended a separate meeting reported being told the same information.

A member of staff at the university's propaganda office said Kwok had "indeed violated some laws and was subject to detention" but refused to provide more details.

Follow-up calls to the department went unanswered.

The police station in Yuexiu District, where Kwok was allegedly arrested last week, also refused to confirm any case details.

"A written notice would be sent to his family within one week after the arrest," an officer said.

Kwok's family were not available for comment. Small Cow and a Hong Kong lawmaker both said the family has not lodged any formal report to Hong Kong's police or immigration authority.

Hong Kong's protests were sparked by a now abandoned plan to allow criminal extradition to the mainland, where the opaque court system is beholden to the communist party.

It has since morphed into a wider anti-government movement.