The head of anti-monarchist campaign group Republic has been arrested at a protest in Trafalgar Square ahead of the Coronation.

Footage shows protesters in "Not My King" t-shirts being detained, including Republic's CEO Graham Smith.

Six demonstrators, including Mr Smith, were arrested by police while unloading placards near the Coronation procession route, the group said.

Republic posted photos of officers taking details from them on Twitter.

"So much for the right to peaceful protest," the group said, adding the officers would not give the reasons for their arrest and confirmed their CEO was among them.

Matt Turnbull, one of those arrested, said the straps holding the placards had been "misconstrued" as something that could be used for locking on.

"To be honest we were never going to be allowed to be a visible force here - they knew we were coming, and they were going to find a way to stop this," he told the BBC.

The BBC later saw Mr Turnbull being led away in handcuffs.

The Metropolitan Police has been approached for comment but has not confirmed the number of those detained.

New legislation passed this week made it illegal to prepare to lock-on to things like street furniture.

On Wednesday, the Metropolitan Police said they would have an "extremely low threshold" for protests during coronation celebrations, adding that demonstrators should expect "swift action".

Republic said hundreds of their placards had been seized and questioned: "Is this democracy?"

"Some ask why we're protesting. It's because we want to use the coronation to change the debate about the monarchy and show that we're not a nation of royalists," the campaign group wrote.

The number of people at the anti-monarchy protest near Trafalgar Square in central London is growing, with the crowds bursting into chants of "Not my King" and "Free Graham Smith".

The protest has been arranged near the route of the Coronation procession, where thousands have gathered to watch the procession from Buckingham Palace to Westminster Abbey ahead of the ceremony this afternoon.

Around 13 Just Stop Oil protesters appear to have been been arrested on the Mall, with a large group from the climate change campaign group seen in handcuffs.

Just Stop Oil has said five demonstrators were also arrested at Downing Street.

A spokeswoman for the group said their plan was "only to display T-shirts and flags", adding: "This is a dystopian nightmare."

Non-profit campaign group Human Rights Watch said the arrests were "something you would expect to see in Moscow not London".

"The reports of people being arrested for peacefully protesting the coronation are incredibly alarming", its UK director Yasmine Ahmed said in a statement.