Anti-knife crime campaigners including bereaved mothers are set to shut down Westminster Bridge as they call for action over recent bloodshed.
The demonstration, dubbed #OperationShutdown, saw supporters initially gather near Downing Street on Wednesday with demands including a Cobra meeting to tackle the surge in violent crime in recent years.
They planned to spend just under two hours in Whitehall before holding a minute's silence near the Houses of Parliament in memory of Pc Keith Palmer, who was murdered by a terrorist while on duty in 2017, and then moving to the bridge.
The action comes after extensive disruption caused by a separate environmental protest - which has seen demonstrations on the Tube network, at Parliament Square, Waterloo Bridge, Oxford Circus and Marble Arch.
The organisers of #OperationShutdown said in a statement:
Grieving parents including Tracey Hanson, whose 21-year-old son Josh was murdered at a bar in west London in 2015, took part.
One of the organisers, Lucy Martindale - whose cousin was fatally stabbed, said:
The group is also calling for an independent investigation into school exclusions, better rehabilitation of prisoners to stop them going on to kill, and for full jail terms to be served for murder and manslaughter.
Some of the country's most senior police officers have already said the highest levels of Government need to come together to deal with the causes of violence.
The Government held an anti-knife crime summit earlier this month and granted an extra £100 million for police in the areas worst affected by knife violence as well as giving officers beefed-up stop and search powers.
But plans to make police officers, teachers and nurses accountable for failing to spot the warning signs of violent crime were criticised.
Chairman of the Police Federation John Apter called the proposals "a bit of an insult", while Royal College of Nursing acting chief executive and general secretary Dame Donna Kinnair said the duty would put increased pressure on frontline staff.
#OperationShutdown organisers said the summit was "non-inclusive, tokenistic and with responsibility being passed onto hard working, already under pressure, nurses, teachers, police and charity front line workers".