The climate strikes sweeping the globe are a "prophetic movement", Christian Aid has said.

Millions of people marched in major cities around the world on Friday, including London, Sydney and New York, calling for an urgent response from global leaders to stop damaging climate change.

The strikes were largely youth-led, with many of the marchers having missed school to take part, but there was also an unprecedented coordinated walk-out in Silicon Valley of tech workers from Microsoft, Google, Amazon, Facebook, and Twitter.

Greta Thunberg, the 16-year-old activist who has become the face of the campaign against climate change, told Congress in a highly anticipated speech that world leaders needed to take the climate crisis more seriously.

She said: "My name is Greta Thunberg, I am 16 years old and I'm from Sweden. I am grateful for being with you here in the USA. A nation that, to many people, is the country of dreams.

"I also have a dream: that governments, political parties and corporations grasp the urgency of the climate and ecological crisis and come together despite their differences – as you would in an emergency – and take the measures required to safeguard the conditions for a dignified life for everybody on earth.

"Because then – we millions of school striking youth – could go back to school."

She continued: "I have a dream that the people in power, as well as the media, start treating this crisis like the existential emergency it is. So that I could go home to my sister and my dogs. Because I miss them.

"In fact I have many dreams. But this is the year 2019. This is not the time and place for dreams. This is the time to wake up. This is the moment in history when we need to be wide awake."

Responding to the global climate strikes, Christian Aid's youth and campaigns manager, Richard Baker, said the climate strikers were "pointing the way".

"The youth climate strikes have been nothing short of prophetic. One seemingly defiant act of protest by a 16-year-old girl in Sweden has grown into a thundering movement for change that is sweeping the globe," he said. 

"The climate strikers are pointing the way, it's now time world leaders and big business take heed and take action. This week at the crucial summit in New York we need to see countries making new pledges to cut their emissions and stronger commitments to help the poorest communities already suffering from the climate crisis."