Activists given a heroes' welcome on their return from a mission to the disputed Diaoyu Islands could yet face prosecution for breaching the city's marine regulations, as new details emerge of police efforts to stop them leaving Hong Kong waters.
Tensions again flared over the islands last night, as a flotilla of 20 boats carrying Japanese nationalists, including lawmakers, set sail in the face of anger from Beijing. Anti-Japanese protests broke out in a number of mainland cities, with hundreds gathering in Xian , Shaanxi province, according to the news portal ytwhw.com 
The Japanese government has refused permission for the group to land on the islands. Organisers said ahead of their departure that they would be holding a ceremony aboard boats moored "within touching distance" of the shore.
Nevertheless, Beijing rebuked Japan over the island visit. "China has made solemn representations to Japan, demanding that it immediately cease actions harming China's territorial sovereignty," the foreign ministry said in a statement.
Hong Kong police disclosed that four marine-police officers boarded the fishing vessel Kai Fung No2 after it left Tsim Sha Tsui last Sunday. They tried to stop it, but found the wheelhouse locked and did not have the equipment or authority to break into it. They got off the vessel when it was 180 metres from international waters.
Police Commissioner Andy Tsang Wai-hung said police would consult the Marine Department on whether to prosecute the activists. The decision would be made after more details were collected, a spokesman for the department said.
The eight activists, including one from Macau and one from the mainland, four boat crew and two journalists were released without charge by Japan on Friday.
The seven who landed on the Diaoyus, which the Japanese call the Senkakus, included the first people from Hong Kong to set foot on the islands in 16 years. Their mission to assert Chinese sovereignty over the islands has won support from the city's public and leaders from across the political spectrum.
Tsang denied the police - who were acting on a Marine Department request - deliberately allowed the protesters to leave. "Our officers tried to get the vessel to stop, but that was not complied with. When the vessel was about to leave Hong Kong waters, the officers decided to withdraw from the vessel," he said.
The officers who boarded the boat were not authorised to use a high level of force to stop the sailing.
Tsang said: "At that time, the officers had only two choices - either to stay on board and leave Hong Kong waters, meaning that they would no longer have the power to exercise their jurisdiction - or to withdraw from the vessel."
The Marine Department said last week that the Kai Fung No2 was registered only for fishing.
Seven activists flew back to Hong Kong on Friday, while the rest of the party is returning on the boat.