Thousands of activists from around the world including Swedish teenager Greta Thunberg hit the streets of Madrid on Friday to demand urgent action on the climate crisis from world leaders attending the COP25 summit in the Spanish capital. "The current world leaders are betraying us and we will not let that happen anymore," the 16-year-old Thunberg said in a fiery speech at the end of the march. "We say 'that's enough". And change is coming, whether you like it or not because we have no other choice," she said. Thunberg, who has become famous for galvanising a generation of youngsters to fight for the environment, was forced to abandon the rally shortly after it began because she was mobbed by supporters and the media. "We would love some action by people in power because people are suffering and dying due to the climate and ecological emergency and we can't wait any longer," she told a news conference before the start of the rally. The march took place in Madrid on the sidelines of the 12-day UN climate conference, which the Chilean capital Santiago had been due to host but was forced to pull out due to deadly anti-government protests. The COP25 summit opened on Monday with a stark warning from the UN about the "utterly inadequate" efforts of the world's major economies to curb carbon pollution. Thunberg -- who refuses to fly because of the carbon emissions involved -- had been en route to Chile when the venue was changed, forcing her to hitch a ride back to Europe on a catamaran. - 'Critical moments' - Demonstrators beat drums and blew whistles as they made their way along the five-kilometre (three-mile) route of the march from Atocha train station. Some carried ballons in the shape of the planet. One group carried a mock coffin representing the death of the environment. Olga Conde, 65, said she had turned out because unless people protest "politicians will not give the courageous responses that are needed now." "They didn't do it before, and if there is no social pressure, they won't do it," she told AFP at the rally. The world's top scientists believe the long-term average temperature rise must be limited to 1.5 degrees Celsius over pre-industrial levels to prevent runaway warming and catastrophic longer term damage. But the level of emissions being released into the atmosphere has risen to an all-time high, triggering global weather hazards from heat waves to intense hurricanes and raging wildfires. "We are in one of the most critical moments of our history and it seems like for the first time we are managing to speak with one voice," Oscar-winning Spanish actor Javier Bardem, a vocal environmental activist, told the crowd. "Without a planet there is no future" and "politicians the earth is dying" were among the signs on display. - 'Insufficient ambition' - The rally was held on the first day of a four-day long weekend when people leave the Spanish capital. Organisers said half a million people had turned out but a spokeswoman for the central government's office in Madrid told AFP police estimated around 15,000 had taken part in the rally. "Speeches are not enough, we need concrete actions that target climate ambition that will resolve these problems," said Estefania Gonzalez, spokeswoman for Civil Society for Climate Action (SCAC) representing more than 150 Chilean and international groups SCAC has been the driving force behind a week-long Social Summit for Climate Action, which begins on Saturday and runs parallel to the COP25, involving hundreds of events, lectures and workshops. In their manifesto, groups involved in the march address a clear message to the decision-makers from the nearly 200 countries attending the UN summit. "We demand that governments participating in COP25 recognise the current climate inaction and state that the insufficient ambition of their agreements will lead the planet to a disastrous global warming scenario," they wrote.