Tens of thousands of protesters marched in Brussels on Sunday to demand urgent action against climate change as world leaders gathered for crunch COP28 talks in the United Arab Emirates.

"No to climate change, yes to system change" and "We want a liveable and healthy planet" were among the slogans on posters and placards.

The crowd braved low temperatures in the Belgian capital to the sound of drums and other musical entertainment following a call by the Climate Coalition, which brings together 90 civil society organisations.

Organisers said 25,000 people attended, while police estimated their number at 20,000.

"This march isn't taking place on any date, it's happening at the start of COP28... we really need international negotiations to move forward," Climate Coalition president Nicolas Van Nuffel told AFP.

"We're in the street with very concrete demands so that we speed up home insulation, increase cycle paths and reinvest in public transport."

Many of the demonstrators sent messages to political leaders, among them Nienke, a student.

"My future is in danger and I really want to live in a safer world... I feel so stressed by what will happen to our planet," she told AFP.

Agnes, a pensioner, felt that relying on politicians was no longer the answer.

"They have not been doing what they should be doing for years... There is no planet B," she said.