Drivers were asked to honk approval when held up by a pollution protest - but were not happy.
Several shouted from car windows at the Extinction Rebellion campaigners who stretched a placard across Collin Street.
One motorist claimed he was getting late for a doctor's appointment and said: "They should stand aside rather than holding everyone up."
He questioned why they were escorted by a police community support officer, adding: "He could be on his beat, saving lives.
"None of this is saving lives. It is wasting public money."
An Extinction Rebellion member said: "That sort of reaction is inevitable. This is such an important issue and governments are doing nothing.
"It is criminal inaction by governments. We understand frustrations of the public and don't want to be creating this.
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"But this matter is wasting lives. People are dying now. There is the extinction of animals, there is a loss of diversity and that is all happening now."
Traffic was stopped for around six minutes on a number of occasions, but the protest lasted from 6pm to 7.30pm. Some protesters spoke to motorists, saying fossil fuels posed the biggest threat to the planet.
They say seas are warming, leading to flooding, famine and wildfires. They say that many animals are in decline - and that insects are also dying out.
"With no insects, ecosystems will collapse and that means no food, clean air or water for us. We face an unprecedented man-made environmental crisis which threatens all our lives," they said.
The protest caused delays to several buses including Navy 1, 3, 48, 49; Green 5, 6, 7, 8, 9, 10, 11; Orange 34 and Turquoise 77, 78, 79/A/B.
Nottingham Travelwise, a city council run Twitter account, said there were "significant delays" around the city centre.
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