The painting emerged unscathed.

The black-clad activists were shown with their hands pressed against the painting in a room of the National Gallery of Victoria in images shared on social media by the Extinction Rebellion movement.

A man wearing a T-shirt with the Extinction Rebellion symbol -- an hourglass in a circle -- stood alongside them.

On the ground at their feet lay a black banner reading "Climate Chaos = War + Famine".

"It is believed three protesters entered the ground level of the gallery before a man and a woman glued themselves to a protective covering of a Picasso painting," said a Victoria Police spokeswoman.

The protesters -- a 49-year-old woman from New South Wales and a 59-year-old man from Melbourne -- were "removed from the painting" more than an hour after their action was believed to have started, the police spokeswoman said.

The pair, along with a 49-year-old Melbourne man, "have been arrested and are assisting police with their enquiries", she said.

Police later said they had been released "pending further inquiries".

The gallery said it had called the police and closed the exhibition to the public during the protest.

"The protesters' hands were safely removed from the perspex with no harm to the work," said a spokesperson for the National Gallery of Victoria.

The 1951 work by Pablo Picasso "shows the horrors of war", Extinction Rebellion Victoria said on its Facebook page.

"Climate breakdown will mean an increase in conflict around the world. Now is the time for everyone and all institutions to stand up for action!" the group said.

The painting was being shown on the final day of the gallery's "The Picasso Century" exhibition.