A huge security operation is being put in place in Bristol for a visit by Greta Thunberg that is expected to attract a crowd of around 25,000, most of them children and young people.

Police and Bristol council officials said there would be significant disruption for the youth climate strike on Friday and warned that they could not be responsible for the care of unsupervised children.

The Guardian heard from children and adults across the country, as far afield as Scotland, who have said they will be attending.

Special coaches are being arranged from London, Cardiff, Exeter, Oxford and Birmingham, and rail and bus stations are gearing up for a huge influx of youngsters keen to take part in the strike and hear Thunberg speak.

At least one Bristol school is expected to close and the numbers will be swollen by thousands of students from the city’s universities and colleges. Police advised people to think carefully before coming into Bristol if they are not taking part in the strike.

Youth striker Kai Damani, 18, a sixth form student, anticipated that up to 30,000 people could attend. Thunberg is due to make a speech on College Green outside city hall before joining a march around the city. She may also visit a mural of herself in the Bedminster area of Bristol.

Damani said the movement was working closely with the police and the city council and would provide its own stewards and wellbeing officers to help. He also said that the risk from coronavirus had been considered but it had not received advice from the authorities to call the strike off.

Bristol city council’s executive director, Mike Jackson, and Supt Andy Bennett, the city’s police area commander, issued a joint statement in which they said they were working to manage public safety and minimise disruption.

But they said: “We know there will be major disruption to the city. We have seen a number of protests over the last year. However, this one will be significantly larger.

“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 encourage those attending, or who are responsible for children who wish to attend, to consider their arrangements carefully and make their own informed decisions. Please do not underestimate the scale of this protest.

“We’re working to ensure Bristol is open as usual, but would urge people to allow extra time for travel and consider whether their journey is necessary.”

Johnny Palmer, founder and managing director of the Bristol company SolCell, which will be providing green energy for the sound system, said his eight-year-old son, Wilfred, would be helping him for the day rather than going to school. “For him, it’s a way of taking positive action rather than suffering from climate change anxiety,” he said.

Ishmael, a 13-year-old from Bristol who is attending, said Thunberg’s visit might inspire those who do not usually attend to turn out: “Lots of us are regular protesters. However, I think this might help others feel like pulling their sleeves up and joining in.”

Willow, a 15-year-old from Gloucestershire, and some of her friends were planning to head to Bristol. She said: “It shows that it takes one person to stand up to something which is not right, and many, many people will stand with them. It shows that time is running out and we are in serious need of change. And it shows that young people should not be underestimated – we have a voice and we are strong.”

A father said his six-year-old wanted to attend. He added: “I might take our 20-month-old too but it depends on his nap.”

Bristol Green party councillor Carla Denyer said: “I fully support the children in taking part in this strike. I have been helping the organisers with some logistics and will be there in person on Friday. Of course education is important, but so is ensuring they have a healthy environment to grow up in.

“Greta Thunberg and the millions of climate strikers she inspired have played such a crucial role in forcing climate change up the agenda and ensuring that politicians and other leaders cannot be seen to be ignoring the issue.”