Scotland Yard has dismissed suggestions from UKIP leader Gerard Batten that water cannons could be deployed at protests.
The Metropolitan Police, which does not own any of the machines, said reports of their use were “baseless and false”.
The UKIP MEP is due to speak at a rally alongside Tommy Robinson against “Theresa May and her traitorous Tories” on Whitehall on Friday afternoon.
Mr Batten later responded that he was "pleased" the police had said there would not be a water cannon at Friday's protests.
He wrote on twitter: "I am pleased to say the Met have said there will no water cannon today. My reports were from a reliable source & they did say they were coming from elsewhere.
"The Met have always been helpful at previous events, for which I am grateful. I repeat my call for a peaceful day all around."
Westminster is expected to see mass protests and counter-protests on the day that Britain was scheduled to leave the EU.
Road closures may be enforced around the area, Transport for London warned.
Westminster Council said it is aware of up to 13 separate scheduled protests and the Metropolitan Police said "appropriate policing plans are in place".
The Metropolitan Police said "appropriate policing plans are in place" to cope with "a number of demonstrations and protests" on Friday.
A spokesman said: "We will always provide a proportionate policing plan to balance the right to peaceful protest, while ensuring that disruption to communities is kept to a minimum.
"To date, protests have been largely peaceful and we have no intelligence at this time to suggest that will change.
"We would like to reassure the public that officers will be ready to respond should any incidents or other spontaneous protests arise."