Tens of thousands of people have taken to the streets of London to demand plans for Brexit are reversed.

The protest is being held just days before Prime Minister Theresa May is expected to trigger Article 50, which will formally start the process of the UK’s withdrawal from the EU.

It also coincides with the 60th anniversary of the signing of the Treaty of Rome, which established the European Economic Community, the EU's precursor.

The march, which was due to set-off from Park Lane at 11am, was delayed by more than an hour, with some suggesting police were unprepared for a turnout being much higher then expected.

Blue and yellow, the colours of the EU, dominated in the crowd and marchers carrying flags representing the countries of the union travelled along Piccadilly, Pall Mall and Whitehall before massing at Parliament Square, where they held a minute of silence for the victims of the Westminster attack.

Many EU nationals living in Britain joined the crowd, represented by the group The 3 Million, but many British citizens and families also marched to protest the Government’s plans for a hard Brexit.

Tens of thousands joined the the Unite for Europe march in central London (Adam Withnall)

Some banners carried a despairing tone, with one declaring Brexit “a EUge mistake”, while another claimed the reality of Brexit “makes me shudder”. One simply declared in big letters: “Tut”.

But among the crowd, there was almost a carnival atmosphere. This was, after all, billed in part as a celebration of the EU's birthday.

In the heart of the throng, a full marching band of drummers kept up a beat and the crowd responded with cheers.

Alastair Campbell was spotted in the crowd taking a selfie with one of the organisers. He also addressed protesters in Parliament Square and said: "We can persuade Theresa May to change her mind, she's already done it once."

Organisers said the march had been organised to make pro-EU voices heard and condemn the leave campaign, which they claim was based on lies and put people’s future at risk for political gain while inciting hate and dividing communities.

A similar event also took place in Edinburgh, which was organised by the Young European Movement. More than 1,000 people are reported to have marched from Waterloo Place in the city centre to the Scottish Parliament, waving EU and Scottish flags.

On its website, Unite for Europe said: ​"We are the 48 per cent, who voted against Brexit and those who were not allowed to vote against it – the young and the EU nationals living, working and paying taxes in the UK. We are outraged by the government’s current direction in dealing with the result of the referendum.”

Making the case for those, who “fundamentally never wanted Brexit”, organisers hope the march will prompt Parliament to listen to those who want the UK to remain a member of the single market, secure the benefits of the EU membership and guarantee that EU citizens already in the country will have a right to stay.

Liberal Democrat leader Tim Farron, one of the speakers at Parliament Square, said he spoke not only for the 48 per cent, who voted remain, but also for those who voted to leave the EU.

"We are here to show solidarity and respect for those who voted leave. We do not believe they wanted this. [Theresa May] does not speak for 52 per cent, she barely speaks for five per cent," he said.

Addressing the crowd, Tottenham MP David Lammy said: "There are lots of people against Brexit in this country, and people are changing their mind.

"We're here because of a lot of anti-immigration rhetoric. We're living in a dictatorship. In democracies, people are always allowed to change their minds. Over the coming months and years we will fight. Nigel Farage wouldn't give up. Labour needs to rediscover its mojo, and quickly."