Road closures are in place across central Bristol ahead of a climate strike march by Greta Thunberg.
The Swedish environmentalist will be at a Bristol Youth Strike 4 Climate (BYS4C) event on College Green, before joining the march through the city.
It is not clear how many people will attend but the event has “grown so large” usual safety measures may prove inadequate, police say.
BYS4C insists adequate safety measures are in place.
Avon and Somerset Police and Bristol City Council have warned of “major disruption” during the event, which is due to start at 11:00 GMT, but stressed they would be “working to ensure Bristol is open as usual”.
More than 20 road closures are in operation from Broadmead and the Bearpit across to Hotwells, Brandon Hill and Park Street.
BYS4C said a similar protest last year attracted 15,000 and it was believed there could be many more people come to hear the 17-year-old speak.
It said it had drafted in more than 80 stewards and has a sectioned-off a “safe zone” for young children and an accessibility area.
Ms Thunberg had originally intended to visit London, but as the area planned for the protest in the capital was too small the organisers recommended Bristol instead.
Two years ago, Ms Thunberg started missing lessons on Fridays to protest outside the Swedish parliament building, in what turned out to be the beginning of a huge environmental movement.
She has become a leading voice for action on climate change, inspiring millions of students to join protests around the world.