The Austrian Hollywood star is in town for an annual conference he organises on climate change.

"Thank you for your commitment to a healthy planet," he says in German in the announcement, broadcast every 30 minutes over the intercom system of all public transport, according to network operator Wiener Linien.

He continues in English that "you're all real climate action heroes, helping to unite in action and terminate pollution".

Schwarzenegger, 77, launched the summit in his native Austria eight years ago to highlight the challenges of climate change.

In his opening speech on Tuesday, Schwarzenegger, who has been an outspoken critic of US President Donald Trump, held back criticism, while calling for "action that makes their (people's) lives better".

"I know that the people are sick and tired of the whining and the complaining and the doom and gloom. They want heroes... We have to win the people over," he said.

This year, former British prime minister Tony Blair is attending, as well as Austrian Prime Minister Christian Stocker and Austrian President Alexander Van der Bellen.

Around 2.4 million people use the city's public transport per day, according to Wiener Linien. The Austrian capital has around two million inhabitants.