Police say they are responding to an incident in Woolwich, south east London, amid reports that a man has been attacked.
Scotland Yard said the incident took place in John Wilson Street, at 14:20 BST.
Pictures on Twitter appear to show three people lying on the ground surrounded by police and paramedics.
London Ambulance Service said an air ambulance and four crews were also at the scene.
A spokesperson could not confirm the number of casualties.
Police said the area around John Wilson Street was shut down.
On its Twitter account, the Metropolitan Police helicopter team called the incident "serious".
A Scotland Yard spokesman said: "Officers have responded to an incident...
"We believe at this stage officers were called to reports of an assault."
David Dixon, a local head teacher, told the BBC News Channel he walked out of the school gates and saw a body lying in the road a short distance away.
He then heard gunshots.
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