Extinction Rebellion protesters have blocked a main route into Bristol that leads to the M32.
Commuters are facing long delays after the campaigners gathered at Cabot Circus amid a series of protests.
The campaign group wants to encourage the government to do more to deal with the issue of climate change.
Traffic has tailed back as far as the M4 junction, about four miles (7km) away. Police have urged motorists to allow extra time for journeys.
There are other Extinction Rebellion protests this week in Cardiff, Glasgow, Leeds and London.
One of the protesters said they had been being forced to act because “governments and local councils aren’t”.
Ben Moss said: “We’re in the middle of a climate emergency and things aren’t changing quick enough, and we’re really sorry for the disruption.
“We recognise it’s an inconvenience and it’s temporary, but it’s nothing compared to the inconvenience that people are going to be experiencing in 20 years – particularly the younger generation who have a massive representation here.”
It comes after Extinction Rebellion staged an 11-day protest in April that brought several parts of London to a standstill.
Some of the 1,000 people arrested during that protest appeared in court this week.