MPs have begun debating controversial proposals to treble university tuition fees as thousands of protesters gathered on the streets of Westminster.

Tension was mounting ahead of the vote in the Commons.

Students, lecturers and activists were preparing to march through central London in a last-ditch protest against the Government's plans to allow English universities to charge up to £9,000 per year in fees from 2012.

Goldsmiths University students have occupied the library since Monday (December 6) in protest over the tuition fess and have also attended the march through London.

There was speculation that up to half of the Liberal Democrat MPs could rebel against the Government.

Liberal Democrat leader Nick Clegg earlier branded opponents of tuition fee rises as "dreamers", and insisted he was not ashamed of backing the plans because he was dealing with "the way the world is".

Meanwhile, Business Secretary Vince Cable said only Father Christmas could satisfy the demands for expensive, popular policies.

The comments came amid frantic efforts to prevent a major split in the coalition on the controversial issue of university funding.

Labour leader Ed Miliband has warned Lib Dems they faced a "day of judgment", calling on them to stick by their pre-election pledge and vote against the increases. "Before the election, they promised families and young people that they would oppose any increase in tuition fees," he said.

"Today it looks like many Lib Dems will break that promise. To abstain in this vote will simply allow the Government to increase tuition fees. I am calling on all MPs - including Lib Dems - to vote against this increase."

A series of last-minute concessions and rumoured offers of Government jobs appear to have failed to head off a significant revolt among the Lib Dem rank and file, with signs that up to half of the party's 57 MPs could vote against the plans.