ENDING: The demo moved to Parliament Square later today
PROTESTS today calling for urgent action against climate change were due to include an ancient magic ceremony.
Demonstrators on the Extinction Rebellion demo also planted trees at 5pm in Parliament Square.
The day of action stopped traffic through central London from 10am by blocking five bridges over the River Thames with human barriers.
Blackfriars, Lambeth, Southwark, Waterloo and Westminster Bridges were closed until 4.30pm after 6,000 people, in an act of "civil disobedience" sat or lay on the roads.
6,000: Demonstrators pack Westminster Bridge today
More than 70 people were arrested for obstructing the highway and bail offences as Met Police officers tied to contain the protest.
The organisation, which is backed by Green Party leader Caroline Lucas, says we are already in the "sixth mass extinction event" and "facing catastrophe."
The blockades, which involved some protestors sitting or laying on the road with arms locked through drain pipes, caused tailbacks throughout central London on many of the approach roads.
ARRESTED: Protestor Bing Jones is taken away
At 4pm in Parliament Square the Extinction Magic: A Grief Ritual for Rebellion event was scheduled.
An Extinction Rebellion spokeswoman said: "At sunset on Rebellion Day witches and magicians responding to the cry of the Earth will lead a ritual for healing in Parliament Square.
"They say 'It is in times of apparent powerlessness that peoples throughout history have rediscovered ancient techniques of infinite potential.'
"A tree-planting ceremony took place at 5pm in Parliament Square with more than than 3,000 Extinction Rebellion protestors present.
"They planted three trees in the centre - plum, apple and evergreen - while singing a sufi song called 'Always in Love'."
Extinction Rebellion is followed by 17,500 people on Twitter.
In posters placed on London bridges in the week before the demo, it warned the world was in the grip of a mass extinction event.
It said: "The science is clear - we are in the sixth mass extinction event and we will face catastrophe if we do not act swiftly and robustly.
HINT: This poster appeared on London Bridge last week
The last mass extinction event - the fifth - was 66 million years ago at the end of the Cretaceous age, when 76 per cent of all species were lost after an asteroid or comet strike.
Met Police supt Waheed Khan said: “The demonstration is having a direct impact on others across London who wish to go about their daily business – and the emergency services from using the bridges to travel around London.
"Given that the organisers failed to engage with police prior to the event, we were unable to work with them around their plan and to make considerations for other Londoners."
GRAFFITI: Protestors spray painted the DEFRA building earlier this week
On Monday, 22 people were arrested after protesters blocked traffic and glued themselves to entry gates at the Department for Business, Energy and Industrial Strategy.
On Wednesday, more activists were detained following action near Downing Street and at the Department for Environment, Food and Rural Affairs in Westminster.
The Extinction Rebellion spokeswoman added: "This is the first time in living memory that a protest group has intentionally and deliberately blocked the five iconic bridges of central London."
DEFIANT: Protestors locked arms through drain pipes
Cecelia B of Extinction Rebellion said: “We are peacefully standing up for the Earth and for humanity. People are dancing and singing and making new friends. This is a joyful rebellion and this is what the future looks like.”
Stephen, a protestor on Southwark Bridge, said: "Extinction Rebellion is in the process of making history. It has been a coordinated attempt to lock down London in a non-violent, peaceful protest for the Earth. Five bridges have been blocked."