Police deployed extra reserves today in a bid to prevent a repeat of ugly scenes that marred the student march two weeks ago.
Scotland Yard said "an increased reserve capability" was on stand-by to deal with any unexpected disorder or outbreaks of violence.
Senior officers, co-ordinating the operation from an underground command centre in Lambeth, have been constantly reviewing the deployment of officers.
They were watching live images of thousands of demonstrators beamed from helicopters hovering over Whitehall.
Members of the Met's territorial support group, officers on horseback and dog handlers were seen at different parts of the protests.
Chief Inspector Jane Connors, of the Met, said officers would take a "robust" approach to violence, with "appropriate and proportionate" policing.
She said: "We're confident in the adaptable policing plan that we have got in place today.
"We have got reserves that will enable us to be flexible and to move resources around to where they are needed to ensure that we don't have the same activity that we had last time."
Jenny Jones, a member of the Metropolitan Police Authority (MPA), watched protesters being penned in by police in Whitehall.
She wrote on Twitter: "Police have kettled demo. Mad. Just makes crowd distressed. Police tell me they are bringing in loos and water."
Officers from the Met's public order intelligence teams have been scouring websites, blogs and online message boards to build a picture of the protests.
Commander Bob Broadhurst, who is responsible for the operation today, appealed for protest groups to contact them with their plans.
But despite the appeal, only two specific events have been put in the official diary for the day.
They were a rally and march organised by Youth Fight for Jobs from Horseguards Avenue, passing Downing Street and ending in Caxton Street.
A separate protest, organised by the Educational Activist Network, was due to take place in Richmond Terrace, Whitehall.