Theresa May has criticised pupils who walked out of class to demand action on climate change.

Thousands of school children are taking part in strikes in 60 towns and cities across the UK, according to organisers Youth Strike 4 Climate.

Demonstrations have taken place in locations such as Edinburgh, Cardiff, Belfast, Cambridge and Brighton, with pupils also descending on Westminster to call for change.

However, Downing Street warned against the "disruption" being caused by the protests.

"Everybody wants young people to be engaged in the issues that affect them most so that we can build a brighter future for all of us," the prime minister's official spokesperson said.

"But it is important to emphasise that disruption increases teacher's workloads and wastes lesson time that teachers have carefully prepared for.

"That time is crucial for young people precisely so that they can develop into the top scientists, engineers and advocates that we need to help tackle this problem."

Outside parliament, protesters carried banners with slogans such as "There is no planet B", "When did the children become the adults?", and "Why should I clean my room when the world is in such a mess".

They want the government to declare a climate emergency and act fast to tackle the problem, inform the public about the severity of the crisis, and change the curriculum to make it an educational priority.

A recent United Nations report warned that unprecedented action was needed to limit global temperature rises to 1.5C above pre-industrial levels.

Anna Taylor, of the UK Student Climate Network, said: "We're running out of time for meaningful change, and that's why we're seeing young people around the world rising up to hold their governments to account on their dismal climate records.

"Unless we take positive action, the future's looking bleak for those of us that have grown up in an era defined by climate change."

The National Association of Head Teachers (NAHT) said it supported the right of young people to express themselves, but it did not condone students missing school to take action.

In a statement, the NAHT said: "While a school leader's role is to ensure children attend school, are kept safe and receive a good quality of education, it is right that individual school leaders can decide how best to respond to any proposed protest by students in their school on Friday."

A number of politicians spoke up in support of the pupils.

Among them was Labour leader Jeremy Corbyn, who posted on Twitter: "Climate change is the greatest threat that we all face but it is the school kids of today whose futures are most on the line.

"They are right to feel let down by the generation before them and it's inspiring to see them making their voice heard today."

Green Party MP Caroline Lucas, who addressed a strike in Brighton, said: "Our children recognise that this is a climate emergency.

"They are striking this week because they know we cannot carry on as normal.

"Teachers work hard to prepare students for their future but right now that future is at serious risk."

Labour's David Lammy said the strikes should "give us hope".

The Tottenham MP wrote on Twitter: "As bleak as politics looks right now, our children, teenagers and young people are full of righteous energy.

"They will reject the bigoted, selfish politics of the past and demand what's right."

Liberal Democrat education spokeswoman and former teacher Layla Moran praised those who have "taken up such an important and principled cause".

The global movement has already seen school strikes in Australia and a number of European countries.

It was inspired by Greta Thunberg, a teenager who protests every Friday outside Sweden's parliament to call on leaders there to tackle climate change.