Protesters marching against plans to increase student tuition fees caused traffic chaos today as they fled police over fears they would be penned in.
Thousands of people scattered through London's West End after they were confronted with massed ranks of police in Whitehall.
They split into separate marches which wound slowly through some of the capital's busiest shopping and business districts, including Piccadilly Circus and the Strand.
The Metropolitan Police denied plans to "kettle" protesters and said officers formed a cordon to guide the march away from gas works.
A spokesman said the march started from Trafalgar Square earlier than agreed, catching police off-guard as they were not ready to escort it.
Two people have been arrested for public order offences but there has been no repeat of high-profile disorder seen during two previous marches.
Police said the spontaneous marches have been largely peaceful but contributed to traffic disruption in the capital caused by snowfall and icy conditions.
The renewed protest, dubbed Day X and the third in recent weeks, came as MPs considered the proposals in an opposition day debate.
Demonstrations, marches, occupations and vigils have also been held in Cambridge, Brighton, Newcastle, Bath, Nottingham, Bristol and Cardiff.
A total of 41 people were arrested last Wednesday after a group of protesters smashed a police van in Whitehall.
Officers surrounded several thousand demonstrators, including many school children, for up to nine hours in a so-called "kettling" operation.
The tactic has been widely criticised as disproportionate and unfair, but police argue it is a legitimate way of stopping troublemakers.
Violence also flared on November 10 after a march by 50,000 students when some protesters smashed windows at the London headquarters of the Conservative Party.
MPs are expected to vote before Christmas on the Government's proposals to increase the cap on tuition fees from £3,375 to as much as £9,000 a year.
In Brighton, all secondary school heads and college principals were sent letters from the council urging them to discourage young people from missing class.
Terry Parkin, of Brighton and Hove City Council, said there was a "significant risk of harm" for schoolchildren attending a march.
In Sheffield, about 200 protesters gathered behind barriers police put up about 30 metres from Deputy Prime Minister Nick Clegg's constituency office.
About 30 officers stood between the crowd and the office in the upmarket Fulwood area of the city as dozens more police waited in surrounding streets.
The Met spokesman said: "The Met police worked with organisers in advance to agree a suitable route from Trafalgar Square down to Parliament Square for a peaceful protest.
"However, today's march set off at an earlier time than agreed. This meant that the march began without a police escort. The police escort was essential due to gas main works on one side of Whitehall.
"As a result, a line of police officers formed a cordon across Whitehall. This line of police officers intended to steer the march to one side of the road and the agreed route. There was never any intention to contain the protesters.
"The march then broke into small groups, travelling in different directions. The march continues peacefully, however, it is causing some disruption for Londoners in the West End, in what are already difficult conditions due to the weather."
The University and College Union (UCU) and the National Union of Students (NUS) outlined plans today to continue their campaign against higher tuition fees.
Both unions have distanced themselves from the violence seen at the recent protests.
Peaceful protests will be held at universities across the country on the day before MPs vote on the issue in the Commons, and there will be a mass lobby of MPs on the day, followed by a rally in Westminster, they said.
A candlelit vigil will be held during the evening of the vote with 9,000 candles representing the potential new fee level.
The day of the vote is not yet known, but the unions said they wanted to put plans in motion now.
UCU general secretary Sally Hunt said: "We need to expose these shabby and devastating proposals and the damage they will do to our universities, colleges and communities. The march of November 10 organised by NUS and UCU has transformed the politics of higher education and placed the coalition's fees policy under heightened scrutiny.
"Further protests today, coupled with hints of ministerial U-turns, mean we must keep the pressure up ahead of the vote and on the day itself. MPs must be left in no doubt of the strength of opposition to these plans and the consequences of voting for them."
NUS president Aaron Porter said: "The joint NUS and UCU march that brought together 50,000 people on November 10 has provided the spur to a new wave of activism and lobbying, placing the Government's policy on fees and student support policy under huge pressure."
He added: "MPs can be left in no doubt as to the widespread public opposition to these plans or of the consequences of steamrollering them through Parliament."
A Met Police spokesman later added that a third arrest has been made at today's protest, for common assault.