Anti-Brexit campaigners have contingency plans for up to 50,000 people attending their march in London on Saturday 25 March, IBTimes UK has learnt.
The demonstration will start at 11:00 and end outside of the Palace of Westminster around 18:00 GMT, just days after the terror attack in and around the historic UK landmark.
A Unite for Europe spokesman defended the group's controversial decision to change the protest's slogan from "Stop Brexit" to "Make Your Voice Heard".
"We're still of the opinion that Brexit shouldn't go ahead, it's very much where we stand," Keiran MacDermott told IBTimes UK.
"We haven't change our aims as such, It's just a change of tone. We had wanted to have a more celebratory tone, but with the terror attack that happened on Wednesday we are again thinking the most appropriate thing is to change the tone again and have a more sombre mood."
MacDermott also explained that the group backs there is a "Democratic representation" at the end of the Brexit talks, which could include a second referendum on the UK's membership of the EU or a free vote for MPs.
He added: "We are just under 20,000 on the Facebook page, but we are quite confident that it is going to be more than that because we know [campaign group] Open Britain have emailed their mailing list, which is half a million, and we got onto quite a few other mailing lists...we are planning for contingencies of up to 50,000 [people]."
The demonstration will coincide with the 60th anniversary of the signing of the Treaty of Rome, the founding document of the EU. The protest will also come just days before Prime Minister Theresa May plans to invoke Article 50 of the Lisbon Treaty and trigger Brexit talks on Wednesday 29 March.
"The Metropolitan Police is working with the organisers of the 'Unite for Europe' Brexit march to facilitate a lawful protest and has a proportionate policing plan in place," a spokesperson for the force said.
"Total Policing is the Met's commitment to be on the streets and in your communities to catch offenders, prevent crime and support victims. We are here for London, working with you to make our capital safer."