An employee at the British Consulate in Hong Kong who was detained on the mainland has been released, Chinese police have said.

Public security authorities in Shenzhen said Simon Cheng Man-kit was released as scheduled after 15 days of administrative detention.

The Luohu public security bureau in Shenzhen, the mainland city neighbouring Hong Kong, made the announcement on its Weibo microblog account.

Hong Kong Protests
Demonstrators link hands across a street in Kowloon (AP)

Mr Cheng was detained for violating mainland Chinese law and “confessed to his illegal acts”, the statement said, without providing further details.

A Hong Kong police officer, speaking on condition of anonymity, confirmed that Mr Cheng had returned to the city.

“Simon is released. Simon is safe,” said Max Chung, organiser of a rally earlier this week to urge the British government to step up efforts to free Mr Cheng.

Hong Kong protests
There have been months of anti-government protests in Hong Kong (AP Photo/Kin Cheung)

“We’ve just managed to talk to him over the phone,” he said, adding that Mr Cheng would answer any further questions but did not say when.

Mr Cheng’s detention stoked tensions in semi-autonomous Hong Kong, which has been rocked by months of anti-government protests.

Mr Cheng, a Scottish Government trade and investment officer, was hired locally and did not have a diplomatic passport.

He was detained after he left for a business trip on the mainland at a Hong Kong high-speed train station.

The Chinese government has said he was detained for violating public order regulations.

Hong Kong Protests
Demonstrators hold up the mobile phone lights as they gather on the Lion Rock mountain against the night view of Hong Kong on Friday night (AP)

Meanwhile, pro-democracy protesters have taken to the streets again, this time to call for the removal of “smart lampposts” that raised fears of increased surveillance.

Activists fear the lampposts in Kowloon could contain cameras and facial recognition software.

Carrying umbrellas in the sweltering heat, they filled a main road in the Kwun Tong district and chanted slogans calling for the government to answer the movement’s demands.

March organizer Ventus Lau said: “Hong Kong people’s private information is already being extradited to China. We have to be very concerned.”

Some protesters set up makeshift barricades on a road outside a police station, facing off with police in riot gear.

Hong Kong’s government-owned subway system operator MTR shut down stations and suspended train service near the protest route, after attacks by Chinese state media accusing it of helping protesters flee in previous protests.

MTR said on Friday that it may close stations near protests under high risk or emergency situations.

The company has until now kept stations open and trains running even when there have been chaotic skirmishes between protesters and police.

Mr Lau said MTR was working with the government to “suppress freedom of expression”.