Planned demonstrations by the far-right English Defence League and anti-fascist groups in London are to be limited by police because of "current tensions".
Scotland Yard said it was taking action under public order laws over the marches "due to concerns of serious public disorder, and disruption to the community".
The protests come in the wake of deadly terrorist attacks on Westminster and London Bridges and more recently close to a mosque in Finsbury Park in the north of the capital.
Superintendent Emma Richards said: "We have made the decision to impose conditions based on current tensions and concerns, information about the intentions of the organisers of these events and intelligence from previous marches held by similar groups.
"We believe the approach is appropriate. We have a duty to ensure that the community in central London can go about their daily business not unduly impacted by demonstrations taking place."
Rules laid down by police mean an EDL march in the heart of London on Saturday will only be allowed to legally take place for 90 minutes from 1pm on Saturday on a route from Charing Cross to a rally on the Victoria Embankment.
A counter-demonstration by Unite Against Fascism (UAF) will be allowed to take place on the Victoria Embankment only between 12.30pm and 3pm. Assembly at any other location is not permitted.
A separate United Against Extremism (UAE) demonstration in the City will be allowed to assemble outside St Paul's Cathedral from noon to 1pm and then march to London Bridge until 2.30pm.
A protest by Britain First is also planned for Birmingham city centre on Saturday.
West Midlands Police said it expected fewer than 200 people to take part in the event centred on Centenary Square.
Chief Superintendent Steve Graham said: "Our understanding is that there will be a counter demonstration in The Eastside area of the city near Millennium Point.
"Ensuring there is no crossover of those with opposing views, and minimising the risk of any potential flashpoints, is a key element of our event planning.
"Trained police negotiators have been in regular contact with both the Britain First and counter demonstrators to identify their intentions and stress the importance that their members protest peacefully.
"However, we will have a highly visible police presence on the ground and sufficient police resources on standby should there be any need to deploy extra officers."