Police were called to the Tate Modern after a woman was seen scaling the 10th floor viewing platform of the buidling.
Metropolitan Police officers and the London Fire Brigade arrived at the the Blavatnik Building at 1.30pm this afternoon.
Witnesses said the woman had climbed over the railings onto the platform - the same area a six-year-old boy was thrown from - and had to be talked down by specialist officers.
Officers later confirmed the incident had been stood down and said the woman would now be given a mental health assesment.
MPS Southwark earlier wrote on Twitter : "Police on scene with LAS and LFB at Tate Modern gallery in #Southwark after concerns for the welfare of a woman.
"Officers are speaking to her. Road closures in place."
The back of the building was partially evacuated, witnesses said.
Revd Dr Ayla Lepine said on Twitter: "I’m at @Tate now. The area is surrounded by police. She’s with someone who may be able to help. Many prayers. Lord, have mercy."
Another witness said: "Someone is at top of Tate modern, lunchtime walk not sure if they’re threatening to jump! They’ve climbed over the railings about 10 storeys Police are on site and removed the lunchtime folk."
One witness said that multiple bystanders stopped to take photos of the incident.
She said: "People are stopping to watch and take photos. TAKE PHOTOS??? why. The poor woman. Police have shut the road and are on hand talking to her."
The Tate Modern confirmed that there would be police and drone activity in the area due to to an unrelated incident that happened earlier this month.
On August 5, a six-year-old boy from France was critically injured after being thrown 100ft from the same viewing platform .
A 17-year-old boy was arrested and charged with murder in connection with the incident.
