Three climate activists on Friday threw soup over two Vincent van Gogh paintings at the National Gallery in London, just hours after two others were jailed for committing the same act two years ago.

The gallery said "a soup-like substance" had been thrown over "Sunflowers" (1888) and "Sunflowers" (1889) and that three people had been arrested.

The Just Stop Oil group (JSO) said it had staged the "sign of defiance" after two of its activists were jailed earlier on Friday for throwing soup at the "Sunflowers" (1888) painting in October 2022.

The attacks are the latest in a series of protests by the group, which seeks an end to the extraction and burning of oil, coal, and gas, which have prompted an increasingly hard line from the authorities.

Phoebe Plummer, 23, and Anna Holland, 22, were convicted in July of criminal damage after pouring tomato soup over the screen protecting the still-life masterpiece. Plummer was on Friday jailed for two years and Holland for 20 months. Both had pleaded not guilty.

Greenpeace UK's co-executive director, Will McCallum, called the sentences "a draconian and disproportionate punishment for a protest that caused minor damage to a picture frame."

Sentencing the pair, Judge Christopher Hehir said the painting could have been "seriously damaged or even destroyed."

"Soup might have seeped through the glass. You couldn't have cared less if the painting was damaged or not," he added.

"You had no right to do what you did to 'Sunflowers'."

Supporters of the pair gathered outside Southwark Crown Court in south London, holding banners calling them "political prisoners."

But Hehir dismissed the argument. "You think your beliefs entitle you to do anything," he told Plummer.

"The suggestion that you and others like you in a democracy are political prisoners is ludicrous, offensive and idiotic."

The National Gallery said the jailed protesters caused around £10,000 (US$13,420) in damage to the frame but the painting itself was protected by a screen and was unharmed.

Holland and Plummer also glued themselves to the gallery wall during their protest.

Plummer has already spent 58 days on remand in prison for another protest at London's Heathrow Airport in July.

Greenpeace UK's McCallum called the sentencing "another grim milestone in the ongoing crackdown on peaceful protest."

"Protest is by its nature inconvenient and occasionally messy. These defendants do not deserve to spend years behind bars for standing up for a liveable planet."

Holland said before she was sentenced that they would not be deterred by prison sentences.

JSO named the supporters in Friday's incident as Phil Green, 24, Ludi Simpson, 71, and Mary Patricia Somerville, 77, confirming they had thrown soup on the art works.

"We will be held accountable for our actions today, and we will face the full force of the law," said Simpson.

"When will the fossil fuel executives and the politicians they've bought be held accountable for the criminal damage that they are imposing on every living thing?" she asked.

The gallery said the incident occurred at 2:30 pm (1330 GMT) and that the paintings were found to be unharmed after being examined by a conservator.

"We are aiming to reopen the exhibition as soon as possible," it added.

Just Stop Oil has staged a number of high-profile stunts in recent years.

In July 2022, protesters glued themselves to the frame of John Constable's pastoral masterpiece "The Hay Wain," also in the National Gallery.

Five activists, including the climate group's founder, were given between four and five years in jail in June for conspiring to plan protests that blocked the M25 motorway surrounding London.

JSO activists were also arrested as they intended to protest at King Charles III's coronation in May last year. The group called the detentions "massive authoritarian overreach." (AFP)