Extinction Rebellion protesters have blocked a main route into Bristol that leads to the M32.
Commuters faced 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 tailed back as far as the M4 junction, about four miles (7km) away. Police said 16 people had been arrested.
A man said he missed being at his father's bedside when he died at Bristol Royal Infirmary earlier because of the "mayhem" caused by the protest.
Avon and Somerset Police said some of the protesters had "crossed the line" and arrests were made after some glued themselves to a bathtub.
Police and crime commissioner Sue Mountstevens warned the operation would cost £300,000.
The road was announced clear at 16:49 BST.
She said it had "impacted on many residents who need to get to work, pick up kids or get to hospital appointments".
During other Extinction Rebellion protests this week in Cardiff, Glasgow, Leeds and London activists used boats to block traffic.
One of the protesters in Bristol said they had been 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."
Police said a group of about 30 protesters initially stopped traffic at about 06:50 BST and some had glued themselves to a bath "on a middle lane section of the motorway".
Motorists using the M32 have been advised to expect further delays.
Ch Insp Mark Runacres said: "Throughout the protest we've been balancing our duties...to allow peaceful protests to take place while balancing that with the need to reduce the amount of disruption to the public and keep people safe.
"The action taken by some of the protesters this morning has crossed the line and is a situation we can no longer accept.
"We were clear that we would take robust action to maintain public safety and that's what we're doing."
Extinction Rebellion Bristol tweeted the roadblock had been suspended temporarily to allow a woman who was in labour through.
Blockade being temporarily dropped to let through a woman in labour.— 🌞Extinction Rebellion Bristol🌞 (@XRBristol) July 17, 2019
Safety is a huge concern to us. We do this because our hearts are breaking for those already dying & suffering in the Global South & those who will feel feel the consequences here in the near future #ActNow pic.twitter.com/nxx4P8i6dY
Bristol's elected mayor, Marvin Rees, said the protesters had "stepped over the line" and made a "tactical error" by blocking the road.
"As a city we did have a level of understanding with them of the action they would be taking this week, and it did not include the M32," he said.
The scale of the protests has been criticised by some residents on social media.
Extinction Rebellion not very well thought out blocking the M32 into Bristol and right by the hospital routes. Much as I agree with action on climate change, this risks annoying people who have to work in hospital, daily attending cancer patients, people visiting the sick etc.— Sarah Ambe 🇪🇺 (@sarahambe70) July 16, 2019
Others have complained that traffic jams caused by the protests could lead to air pollution.
This is the traffic coming off the M32 into our residential area in Bristol. It’s a massive pollution filled car park directly outside our homes with fumes coming in the window. I’m disgusted. How dare you do this. pic.twitter.com/wMXXYRMwwQ— Chopsy Bristol (@ChopsyBristol) July 17, 2019
At the scene
By Liz Beacon, BBC Points West
There are still four people glued to a pink bath tub which is blocking the main road into Bristol. They're under a gazebo surrounded by about 35 police officers.
It's not clear how long it will take to free them.
The protesters have received a mixed response from people driving past, some beeping in support, and others shouting in opposition.
Generally the mood is calm and respectful.
As each arrest has been made the protesters have chanted: "We love you, we love you, and we thank you for your sacrifice."
Officers have been drafted in from other forces across the west, including Dorset, Wiltshire and Devon and Cornwall.
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.