It's a very wonderful action today. We can't any longer allow governments to do this so we have to make it clear that there is no more time.
Actress Juliet Stevenson is among the celebrities taking part in the Extinction Rebellion protests currently taking place at Trafalgar Square as part of the "international rebellion", along with actress Ruby Wax and models Daisy Lowe and Arizona Muse.
The Metropolitan Police said that as of 12.30pm today, they had made 135 arrests as thousands of activists poured onto the streets of London, shutting down government buildings and nearby roads.
Ms Stevenson, the Truly Madly Deeply star, said: "It's a very wonderful action today. We can't any longer allow governments to do this so we have to make it clear that there is no more time.
"There's a long tradition in this country of people saying governments are not acting, we have to make them realise how urgent this is. I'm optimistic about the energy there is amongst people to act but I'm not hugely optimistic about government stepping up to the plate.
"They're [the government] talking about 2050 and scientists have said we have 12 years before we're in a place where the climate is irreversibly damaging our planet and we won't be able to repair or fix it. We need to make them realise that time is not on our side at the moment."
Ms Stevenson said she was delighted to see so much engagement from young people and that her own son was at the protests and working for Extinction Rebellion.
The Metropolitan Police said that by 8am on Monday there had been 21 arrests in connection with the Extinction Rebellion protests.
The arrests are in addition to those over the weekend, with eight people arrested on Saturday - seven on suspicion of conspiracy to cause public nuisance, and the eighth on suspicion of obstructing police. All those arrested on Saturday have been released under investigation.
On Sunday one woman and two men were arrested on suspicion of conspiracy to cause public nuisance. The woman has been released under investigation, the men remain in custody, the police said.
Extinction Rebellion protesters playing steel drums marched from Parliament Square to Trafalgar Square where they plan to kick off two weeks of disruption in the city.
A 25-year-old teacher from London who took the week off to join the protests said: "The plan is to shut down the whole of London. The issue of climate change is an issue for everyone, and it's clear we can't leave it to the politicians.
"I booked the week off work for this. The school kids are out protesting too. Everyone is."
Police are searching anyone suspected of carrying paint.
Extinction Rebellion said protesters from the XR Peace group arrested as they blocked Victoria Embankment outside the Ministry of Defence included 81-year-old Sarah Lasenby, a Quaker and retired social worker from Oxford.
She said: "For 21 years my main concern has been to help get rid of UK nuclear weapons. I am still keen to do this but once I came across XR I was so relieved to have something I could do about the ghastly state we have got our planet in."
And she said: "The whole thing is so urgent that it is imperative the Government should take serious actions and put pressure on other states and Global Powers to radically reduce the use of fossil fuels even if this means we need to reduce our comfort at home and so much flying."
Protesters have been carried away by police officers on Horse Guards Road. An onlooker said he saw at least five people arrested after they refused to let a van through the road.
Video of the moment shows people being carried off to chants of "we love you" from other protesters. Protesters have blocked Victoria Street and are now lying under a van while surrounded by police.
An onlooker said: "Looks like lots of mini protests rather than a centralised area, huge range of people - if anything mainly middle-aged people."
Extinction Rebellion protester Caroline Hartnell, 69, from London, said she will be attending the protests every day.
She said: "This is the start of two weeks of action. We are going to be surrounding all the Government ministries.
"We are going to be putting pressure on them what they are going to do mend the climate emergency, because we are running out of time.
"I have seven grandchildren and the youngest is three. I feel passionately for them (and worry) there is not going to be a world for them to live in."
Oxford University student Fergus Green from St Albans was also amid the growing crowd of drummers, stewards and flag-waving protesters planning to "shut down Westminster".
He said: "People are coming from all over the country. I'm here to force the Government to take action on climate change. I'm a student, I should be at uni - my term has started. But I'm here to take action. People are missing work and school to be here."
Police have managed to break into an Extinction Rebellion van on Westminster Bridge and have arrested the driver, an onlooker said.
James Bickerton told the PA news agency: "Protesters blocked Westminster Bridge with two vans and started building a stage. Police have broken into one van and surrounded the other (at least two arrests) but still a lot of protesters blocking the bridge."
Two groups of protesters have blocked Westminster Bridge. One man climbed onto a van parked in the middle of the bridge, surrounded by police officers, and lay down on the roof, while other protesters did a yoga class in the rain.
Victoria Embankment remained closed outside the Ministry of Defence.
And Victoria Street close to Westminster Abbey, and side roads by Methodist Central Hall, are being held by another group of protesters with banners that read "tell the truth" and "no coal mines, no fracking".
On the first day of the two-week international protest, Extinction Rebellion protesters have made their way to Whitehall waving "XR" logo flags and marched with a steel drum band.
Protesters have said they plan to "shut down the whole of Westminster" and surround the ministries.
Hundreds have already filled Trafalgar Square and plan to shut down roads around Parliament Square and Whitehall.
Police are stationed outside the Houses of Parliament and have already made 21 arrests.
Two protesters have mounted a Land Rover and trailer at Trafalgar Square roundabout, one wearing a gas mask and trench coat and the other with an XR flag and a sign reading "stop ecoside".
Police have surrounded the vehicle but have not moved the protesters.
Extinction Rebellion protesters are now performing yoga on Westminster bridge. James Bickerton said: "Protesters on Westminster Bridge have lit an incense candle and got mats out for yoga. It's all gone very rainbow rhythms."
Protesters are lying under a trailer parked in the middle of the road at Trafalgar Square. Others stacked a pile of items including a kitchen sink, pans, and camp chairs beside them on the pavement.
This article is based on copy provided by PA.