Swedish climate activist Greta Thunberg has issued a rallying cry to would-be protesters, saying “it’s never too late” to pioneer the fight for the planet.

Speaking in London, the 16-year-old said humanity had reached a crossroads as she addressed hundreds of Extinction Rebellion (XR) supporters in Marble Arch.

After taking to the stage, the teenager told the Press Association she had a message for people who may have only started paying attention for the first time after the demonstrations of the past week.

She said: “Many people are scared and feel bad – ‘I was not the one of first people to do that; I am behind now; I’m only going to look silly because I did not start before’ – but what they don’t realise is that it’s so few that have actually realised what is going on.

“So if you start now, you are going to be one of the pioneers, so it’s never too late but especially now, the struggle has barely begun. It’s only the beginning.”

Asked how she felt about the future, Miss Thunberg said: “I am very concerned but I mean it doesn’t matter if you are concerned or if you are hopeful, you still have to do everything, because being concerned or being scared is not an excuse for not doing anything really.

“It’s up to us, we can still turn this around.”

The teenager travelled to London by train and will stay in the capital until Tuesday evening.

She is due to meet with Labour leader Jeremy Corbyn and the Green Party’s Caroline Lucas, and will also speak in Parliament.

However, it is understood that Downing Street has not responded to her requests to meet Prime Minister Theresa May.

Asked if she expected the XR protests to last as long as they have, Miss Thunberg said: “No I didn’t, I thought it would be for more like one, two or three days, but I mean there are loads of them still here….so yes I really admire them.”

She added: “I think it’s wrong that this is happening, this existential crisis, and that people have to do these things.

“Civil disobedience – we shouldn’t have to do this.”