Ms Emily Lau, deputy chairman of the Democratic Party, said the group was stopped by immigration officials and detained for half an hour, then sent back to Hong Kong on a ferry.

Macau's public security police said they are responsible for turning away those who fail to meet immigration rules, but did not say whether the activists or lawmakers had violated any provisions.

The pro-democrats were planning to march against a proposed national security law introduced by the Macau government last week.

The proposed law would toughen penalties for crimes such as treason and subversion.

Several hundred other people marched yesterday, the ninth anniversary of Macau's handover.

A similar law proposed in Hong Kong in 2003 sparked huge public dissatisfaction, including a march by 500,000 people worried about curbs on freedom of speech. The Hong Kong government was forced to abandon it.

Ms Lau said pro-democrats were worried the Macau Bill was a precursor to its reintroduction in Hong Kong.

AFP, AP