Prominent Black Lives Matter activist Sasha Johnson is in intensive care and fighting for her life after being shot in the head, her political party said.
A vigil was held on Monday afternoon in a south London park, near where she was shot.
London's Metropolitan Police have not confirmed her identity but said a woman, 27, was shot shortly before 3am at a gathering in Southwark, London.
Officers said the woman was taken to hospital with life-threatening injuries. They appealed for witnesses and said her family had been informed.
On Instagram, the Taking the Initiative Party said Ms Johnson is a mother of two and a "powerful voice", who had always fought for black people and the injustices that surround the community.
"Let's all come together and pray for Sasha, pray for her recovery and show our support to her family and loved ones", it said.
"She is currently in intensive care and in a critical condition. The attack happened in the early hours of this morning, following numerous death threats as a result of her activism," the post said.
"Sasha has always been actively fighting for black people and the injustices that surround the black community, as well as being both a member of BLM and a member of Taking the Initiative Party's Executive Leadership Committee.
"Sasha is also a mother of three and a strong, powerful voice for our people and our community."
Police said the shot was fired "in the vicinity of a house where a party was taking place" and a number of people may have been in the area at the time.
"While the investigation is at an early stage, there is nothing to suggest that the woman who was shot was the subject of a targeted attack or that she had received any credible threats against her prior to this incident," the Met Police said.
Detectives from the Met's Specialist Crime Command have been conducting inquiries at the scene in Consort Road and the surrounding area.
"This was a shocking incident that has left a young woman with very serious injuries," Det Chief Insp Jimi Tele said.
"Our thoughts are with her family who are being provided with support at this terribly difficult time."
Mr Tele said detectives were making progress but needed the public's help.
"If you saw anything suspicious in the Consort Road area in the early hours of Sunday morning or if you have heard information since that could help detectives, it is crucial that you get in touch."
Ms Johnson, a graduate of Oxford Brookes University, has been a leading figure in the Black Lives Matter movement in the UK.
She was one of the organisers of the last year's Million People March to protest against systemic racism in the UK. During the demonstration, hundreds of people took to the streets of west London.
She said she hoped to "empower the community to strive for better".