The global wave of school strikes for the climate over the past year has “achieved nothing” because greenhouse gas emissions have continued to rise, Greta Thunberg has told activists at UN climate talks in Madrid.

Thousands of young people were expected to gather at the UN climate conference and in the streets of the Spanish capital on Friday to protest against the lack of progress in tackling the climate emergency, as officials from more than 190 countries wrangled over the niceties of wording in documents related to the Paris accord.

In the four years since the landmark agreement was signed, greenhouse gas emissions have risen by 4% and the talks this year are not expected to produce new commitments on carbon from the world’s biggest emitters.

Thunberg, whose solo protest in Sweden in 2018 has since snowballed into a global movement, spoke at a press conference before a march through the centre of Madrid. She said that although schoolchildren had been striking around the world, this “has not translated into action” from governments.

Facebook Twitter Pinterest South American indigenous people attend a protest in Madrid demanding action against climate change. Photograph: Rodrigo Jimenez/EPA

“I’m just an activist and we need more activists,” she said. “Some people are afraid to change – they try so desperately to silence us.”

Thunberg expressed hope for the UN negotiations but doubted whether governments had got the message, and warned the world could not afford continued inaction.

“I sincerely hope COP25 will reach something concrete and increase awareness among people, and that world leaders and people in power grasp the urgency of the climate crisis, because right now it does not seem that they are,” she said.

Although young people would keep striking, Thunberg said, they wanted to stop – if governments made credible promises and showed a willingness to act.

“We can’t go on like this; it is not sustainable that children skip school and we don’t want to continue – we would love some action from the people in power. People are suffering and dying today. We can’t wait any longer,” she said.

The march was scheduled to coincide with protests and youth climate strikes around the world. In the US, Bernie Sanders and Jane Fonda were among the politicians and celebrities planning to join in.

Play Video 15:08 Inside the mission to create an army of Greta Thunbergs – video

As well as the march and a sit-down protest in the conference centre, there were shows of international solidarity among young people from around the world, including a picnic in a central Madrid park. The conference centre was flooded with hundreds of schoolchildren accompanied by their parents, many with babies in prams, who were kept separate from the rooms where negotiators were working on a draft text to clarify aspects of the Paris agreement.

Young people voiced their frustration at protests inside and outside the conference centre on the outskirts of Madrid.

Brianna Fruean from Samoa, speaking for the Pacific Climate Warriors, told the conference: “World leaders need to know that people like me are watching them. The text we put down today on paper at COP is what our future will look like.”

Many of the young people joining the conference from developing nations around the world bore personal witness to suffering they had experienced or seen.

“I’ve had typhoid. I’ve had malaria. My grandmother died from cholera. I know what I’m talking about,” said Jimmy Fénelon, the national coordinator of the Caribbean Youth Environmental Network in Haiti. “We need to raise awareness among young people. We can get them to work together and send a strong message.”

Renae Baptiste, also from CYEN, said: “For us, climate change is no longer a concept or theory, it’s our new reality. It’s affecting our lives now.”

The activist Miguel van der Velden said: “These things are not games. They’re getting worse. They’re affecting millions of people around the world. I come here because I have hope that we can work together.”

• This article was updated on 9 December 2019 to correct the year Greta Thunberg’s school strike started – it was 2018, not 2017.