Five men, aged 18 to 22, were arrested on suspicion of arson on Monday morning and a 24-year-old man was arrested later in the evening, a police spokeswoman said. The six, who included four college students, were under investigation but no charges had yet been laid.

The small blast followed hours after a contentious copyright bill, which opponents say will restrict freedom of expression, had been listed for debate but was adjourned when not enough legislators turned up. No one was injured.

The South China Morning Post said two of the six were members of a pro-Hong Kong independence group, Valiant Frontier, which had rallied online against the controversial copyright bill.

The bill aims to amend the city's copyright laws by extending the protection of copyright owners to the Internet. Opponents of the copyright legislation worry it could restrict freedom of speech, creativity and political satire in a city chaffing at what many residents see as a gradual whittling away of civil liberties by the Beijing government.

"Hong Kong residents have the right to express their opinion in a legal, peaceful, and rational way, but we can not accept any form of expression which is violent, against the law, and hurting others while causing damage to property," Hong Kong leader Leung Chun-ying told reporters late on Monday.

"The SAR government and police will seriously deal with any illegal matter," Leung said as he left for Beijing.

Last year, tens of thousands of protesters blocked major roads for weeks in a push for full democracy, presenting Beijing with one of its most serious challenges in years.