Some wore school uniform, with ties askew in St Trinian’s fashion, others donned face paint, sparkly jackets and DM boots. The youngest clutched a parent’s hand as people gathered in the sunshine in Parliament Square in London, a few metres from the politicians they say are letting down a generation.

They carried homemade placards, with slogans full of humour, passion and hope that the voices of thousands of children and young people would be heard.

“March now – or swim later”, “I’ve seen smarter cabinets in Ikea” and “denial is not a policy” read the banners, as chants for action on climate change grew and strengthened with the passing hours into a deafening roar. “What do we want? Climate action! When do we want it? Now!”

Across the UK it was a day for thousands who are not normally heard, as children took time out of their lessons to attend the strike.

“As students we don’t have the vote, and it is really unfair because this is going to impact on us the most. It is our future,” said Evie Baldwin, 15, from north London.

In Manchester, the students came with handwritten notes from their parents giving authorisation for their photos to be taken, and bullet points to explain why they felt compelled to strike.

Thousands of pupils from schools, colleges and universities have attended the nationwide climate change strike. Photograph: Guy Smallman/Getty Images

There was a party atmosphere outside the central library, as students played Where is the Love? by the Black Eyed Peas and I Want It That Way by the Backstreet Boys.

They cheered as a band played Joni Mitchell’s Big Yellow Taxi, with its ecological refrain: “Don’t it always seem to go, that you don’t know what you’ve got till it’s gone/They paved paradise, put up a parking lot.”

Lillia Adetoro, nine, told Manchester demonstrators: “Scientists across Europe say we have 12 years to get this right. The technology is there. The solutions are there. Brilliant minds across the world have been working on this for decades. And what they have said has been ignored.”

Children who had been told not to miss school defiantly attended the rallies anyway. Ten-year-old Hettie Ainsworth, in Brighton, had not been given permission by her primary school to join the protest, but such was her passion that her parents let her attend anyway.

Students are urging the government to declare a climate emergency and take action on climate change. Photograph: Guy Smallman/Getty Images

To her, the issue was personal. She said: “The government isn’t doing enough about it. It’s important because it’s our future and if we don’t start paying attention to climate change, there may not be one.”

There was a range of age groups present, from people in their 60s, to children as young as four and five, running around in superhero costumes. Swedish 15-year-old Greta Thunberg was frequently cited as an inspiration by adults and children.

One group called Fridays for Future Manchester, which has been meeting outside the library every week for more than two months, stood with a banner painted with Thunberg’s face on it. Protesters from Extinction Rebellion and Campaign against Climate Change were also present.

Similar crowds were seen across the north of England, with organisers assembling in Leeds, Newcastle and Sheffield.

Ed Miliband attended the demonstration with his nine-year-old son, Daniel. Photograph: Guy Smallman/Getty Images

Magid Magid, Sheffield’s lord mayor, who banned Donald Trump from visiting the city last year, said there was no better time to march than now. He said: “It’s as simple as action or extinction … [These students] come across as the fearless advocates in the face of those who have stopped caring about [the climate crisis]. They are literally saving our bloody planet.”

At the clock tower in the centre of Brighton, hundreds of primary, secondary and university students gathered.

As they marched towards the Level park, the crowd swelled. According to organiser Mary-Jane Farrell, more than 1,000 people were in attendance. While adults and supportive parents followed, teenagers in school uniform marched at the front of the crowd, shouting, among other things: “Greens in, blues out” and “Fuck the Tories”.

The procession blocked traffic on Brighton’s main roads, and onlookers clapped and took photos as people passed through.

Protesters say the government is not doing enough to prevent climate change. Photograph: Facundo Arrizabalaga/EPA

Joe Paugler, 16, left his school during break time to join the march. He said he felt the need to raise awareness. “They don’t discuss climate change as much as they should in politics, probably because there’s no money in it,” he said.

Critics of the strike had suggested some students would use the strike as an opportunity to skip school, but most children seemed more excited by the prospect of attending their first protest.

At Level park, crowds cheered for several of the day’s speakers, but the loudest applause was undeniably for the local Green party MP, Caroline Lucas.

Surrounded by children vying to get a selfie with her, she said today marked a real turning point. “It’s children’s futures at stake, and frankly our generation has let them down and certainly the politicians at Westminster have let them down,” she said.

Hundreds of young people gathered across Glasgow, cheered on by MSP Ross Greer, who tweeted: “Just look at these incredible young women, who can’t be older than 13/14, taking action to save the world and their future.”

In London, there were more than 1,000 in attendance by lunchtime. Among them was former Labour leader Ed Miliband with his nine-year-old son, Daniel. “I am here because it is our future, and we need to protect it,” said Daniel.

The demonstration reaches Downing Street. Photograph: Stefan Rousseau/PA

The size of the demonstration took the police by surprise. “I don’t know how many are here, but it’s a lot – much more than we expected,” said one officer.

Several hours in, what had begun as a rally outside the House of Commons turned into a spontaneous running march on Downing Street, as hundreds of teenagers, sprinted along Whitehall to chant their message outside No 10.

As they surged down the road, buses, trucks and taxis, were forced to stop. “Honk your horn,” the teenagers shouted, to blasts from scores of drivers.

Other groups broke away from the main crowd to sit down in the road, forcing Westminster traffic into gridlock for more than two hours. Holding hands, they refused to be moved, as the police deployed mounted officers in an attempt to move them back into Parliament Square.

One officer successfully lifted 15-year-old Alex Cooper off the road – only for her to continue her seated protest moments later. “This is a great turnout,” she said. “We need the government to listen.”

Across the street, among those watching from the sidelines, one woman said: “My 13-year-old daughter is in there somewhere. I am so proud of her.”