Thousands of students brought central London to a standstill yesterday as they angrily protested the government's plan to allow universities to treble tuition fees to (pounds) 3000 a year.

The National Union of Students organised the march and estimated that 31,000 people had converged on the capital from England, Scotland, Wales and Northern Ireland, although police said the number was about 10,000.

Students, teachers, workers and children urged Tony Blair, prime minister, to scrap his plan to introduce top-up fees, carrying banners saying: ''Age 21 and 21 grand in debt'' and ''No tuition fees. No top-up fees.''

Iain Duncan Smith, the embattled opposition leader, who had earlier met with students in a pub to explain his plan to scrap all tuition fees if elected, joined the march with the shadow education secretary Damien Green.

Mr Blair and Charles Clarke, the education secretary, said this week that they were looking at ways of ensuring that the full fee was subsidised for the poorest students once the

higher charges, if endorsed by parliament, come into force in 2006.

Mr Clarke said between 35% and 37% may get the full costs covered, although Mr Blair did not confirm this figure.

The march, which began at the University of London Union and ended with a rally in Trafalgar Square, lasted three hours and was peaceful throughout, police said.

Mandy Telford, president of the National Union of Students, leading the march, said: ''Students are graduating with record levels of debt. In fact, students have never been

poorer.''