Extinction Rebellion has carried out protests in five UK cities, marking the start of what it describes as a “summer uprising”.

The environmental activist group is calling for greater government action against the climate emergency. Monday’s demonstrations – targeting London, Cardiff, Leeds, Bristol and Glasgow – caused disruption to traffic in parts of the cities.

Protests in each city are focusing on a different ecological threat: rising sea levels, floods, wildfires, crop failures and extreme weather. According to Extinction Rebellion, more than 3,000 activists across the country have signed up to participate in acts of civil disobedience this week, a third of those in London.

Extinction Rebellion protesters block the Neville Street Bridge on one of the main approach routes into Leeds. Photograph: Ian Forsyth/Getty Images

The group has installed large coloured boats in each location, branded with the message “Act Now”.

Extinction Rebellion is demanding that the UK government prevents further losses to biodiversity and commits to producing net zero greenhouse gases by 2025.

The group said it would carry out “creative acts of civil disobedience” including blocking roads and bridges, as well as holding educational workshops. The London activists have promised to continue the action for a week.

More than 15 police vans accompanied the crowd outside the Royal Courts of Justice in London, where about 250 protesters blocked the Strand from about 9am to 5pm. The group included children as young as three months old. Police said there had been no arrests.

A spokesperson for Extinction Rebellion said: “After [our campaign in] April, parliament declared an environmental emergency, but no action has been taken on this, so we’re here to remind them that actions speak louder than words.”

The group said it had chosen the Royal Courts of Justice after it was announced last week that more than 1,000 activists who had participated in April’s demonstrations were facing prosecution. Extinction Rebellion disrupted London and other cities with 11 days of protests that it cast as the biggest act of civil disobedience in recent British history.

“We’re here in solidarity with them and climate activists all over the world who are putting their lives on the line for climate justice,” a spokesperson said.

The group occupied the Strand for the duration of the day, with activists sitting on the ground and singing together to stop traffic. In the early evening the group moved on to a camp in a park near Waterloo where they planned to stay overnight.

Protesters from Extinction Rebellion with a boat that has been parked outside the Royal Courts of Justice in London. Photograph: Kirsty O’Connor/PA

Sue Hayman, the shadow environment secretary, addressed the crowd from the boat installation. She praised the pressure that Extinction Rebellion was putting on the government and local councils to highlight the severity of the climate crisis.

“If we’re going to make a real difference, we have to have the political will to do it,” she said. “This is serious, this is a real emergency and we cannot carry on as we are.”

Roc Sandford, 61, said: “I’m involved because I have children and I want them not to starve and die in social collapse. If you look at what scientists are saying, that’s what’s coming.”

Wilf, a 50-year-old teacher who did not give his surname, said: “We’ve all read the science. We know the story. The whole phase of denial is over, and if it takes civil disobedience to make a difference, then so be it.”

A Met police spokesman said the force “absolutely recognise the right for people to protest, but it ought to be recognised that we must balance this with the rights of others to go about their daily lives. We also have to consider policing across London, which significant protest requiring the deployment of large numbers of officers could impact.

“We have been engaged with the organisers to understand their plans but we cannot tolerate behaviour that crosses a criminal threshold, or causes significant disruption to communities across the capital.”

Bristol Bridge was closed because of the protests, with activists planning to occupy it for four days. South Wales police said roads in Cardiff city centre were also blocked, where activists conducted a people’s assembly to discuss the climate emergency. Protesters locked themselves to the boats using metal tubes, and invited local residents to join the group and enjoy their “car free city”.

Another boat was installed on Trongate in Glasgow with about 250 activists taking part.

In Leeds, protesters targeted the financial district to protest against the sector’s investment in fossil fuels. They also set up tents on Victoria Bridge, one of the main transport networks over the River Aire, and said they planned to remain there overnight.

Alex Evans, 43, said: “My eldest child is nine years old and for each of those nine years I’ve watched her future get steadily worse while everyone waits for everyone else to do something on climate change. Now we’re out of time and we can see climate breakdown all around us. Enough is enough: it’s time to act now.”