A group of protesters taking part in a mass demonstration against higher tuition fees broke away from the main route and erected around 25 pop-up tents at the base of Nelson's column in Trafalgar Square on Wednesday.
Some 5,000 marchers were taking part in the protest, which was being routed away from the other main Occupy encampment outside St Paul's Cathedral.
A heavy police presence, aimed at preventing any repeat of the violence that marked a similar student protest last year, was keeping the marchers away from landmark City sites like the Bank of England and the London Stock Exchange.
Police have also insisted that the culmination of the march, a planned rally at London Wall in the City financial district, should last no longer than two hours.
There were no immediate reports of any violence or disorder in the first two hours of the march.