But union spokeswoman Elaine Hui Sio-ieng insisted they would not seek prior approval.

'It's a basic civil right to organise a peaceful demonstration,' she said. 'Why should one have to apply for such rights?'

The Public Order Ordinance requires police authorisation for public assemblies of more than 50 people and marches of more than 30. Organisers have to give police seven days' notice of a protest, or risk facing a maximum five-year jail term.

Ms Hui accused the Government of political suppression and restricting freedom of expression.

But the police spokesman said: 'While respecting people's rights and wishes to gather and express their views, it is the duty of the police to prepare for these events so as to ensure public safety, public order and to minimise the inconvenience caused to other members of the public.'

He said police approached the organisers on July 23 and repeatedly advised them to adopt proper and lawful procedures in preparation for the march.

Officers had also made suggestions to the students as to how they could ensure the procession was lawful and peacefully conducted.

'However, the police regret that the organisers have not handed in their written notification,' the spokesman said. 'They were unwilling to discuss with the police details of the event.'

In October, the Department of Justice decided not to proceed with charges against 16 protesters arrested over a right-of-abode rally on June 26 last year outside the Central Government Offices.

The department also dropped charges against five student leaders protesting over tuition fees on April 20 last year.