The 17-year-old activist is due to address a crowd of school pupils and university students on 28 February before joining a march around the city to highlight the danger to the planet from climate change.
But Avon and Somerset Police are warning parents that the event, organised by Bristol Youth Strike 4 Climate, has “grown so large” that it could be unsafe for their children.
The force said there was potential for protesters to be hurt from either tripping, falling or being crushed by the crowds and stressed that “parents are responsible for their children”.
At least two schools in Bristol will close on Friday because of their proximity to College Green, where Greta is due to make a speech. Some school exams have been postponed because of the closure.
A similar protest in September last year attracted 15,000 young people – but organisers say as many as 30,000 could turn out on Friday to hear the Swedish teenager speak.
Aden Harris, 14, one of the organisers, told The Independent: “I think most people will be Bristolians who have been looking at Greta for the past year and thinking what an inspirational young woman she is. It will be mostly young people as we are the people this issue affects and we do care really strongly about this.”
Greta started missing lessons most Fridays to protest outside the Swedish parliament building, in what turned out to be the beginning of a huge global environmental movement.
For more than a year, students across the UK have been walking out of classrooms on one Friday a month to demand action on climate change.
Bristol Youth Strike 4 Climate campaigners, who already took part in nationwide school strikes on 13 February, were only told last week that Greta would be coming to Bristol for a protest.
Aden said: “I think she chose Bristol because it is such a wonderful city. We have a big environmental presence and lots of pressure groups. And in September we had the second-largest strike in the country.
“Greta coming will force people to look at us again and think: ‘They are still here and are going strong.’ I think it will increase our numbers for the future and give us a leg to stand on when we make demands.”
Police and Bristol City Council have warned of “major disruption” due to the event and said people from across the UK were planning to travel to the city for the protest.
Some roads will be closed for most of the working day in order to reduce the risk of harm as they anticipate “large numbers of children in the city”.
— Greta Thunberg (@GretaThunberg)February 22, 2020
In a joint statement, Mike Jackson, Bristol city council’s executive director, and Superintendent Andy Bennett, the city’s police area commander, said: “We have seen a number of protests over the last year. However, this one will be significantly larger so we want to ensure that anyone planning to attend is prepared and able to make their own safety and safeguarding arrangements.
”Parents are responsible for their children. The council and police are not responsible for unsupervised children. The event has grown so large that the usual controls, stewarding and safety measures that are routinely put in place by the teenage Youth Strike 4 Climate organisers may not be adequate, especially for primary school children and people with disabilities.
“We would therefore encourage those attending, or who are responsible for children who wish to attend, to consider their arrangements carefully and make their own informed decisions.”