Armed officers were investigating reports that shots were fired at a police patrol outside a busy railway station today.
One officer suffered a cut to his thigh outside Waterloo station in central London, shortly after 3pm.
But a Metropolitan Police spokesman said it was not yet known how he suffered the injury.
The spokesman said: "Police were called at 3.02pm to reports of shots fired at officers at The Cut, SE1.
"Local officers and firearms officers attended. A male officer has sustained a cut to his leg.
"We do not know how the injury has been sustained at this stage.
"He has been taken to a south London hospital as a precaution. No arrests have been made. Inquiries continue."
The Cut is a busy shopping street which runs between Waterloo railway station and Southwark Tube station, a short distance from the River Thames.
Sources said the officer suffered a minor graze-type injury and it was not known what caused it.
A London Ambulance Service spokesman said a second person was also taken to hospital. They also had minor injuries.
The spokesman said he could not confirm that the second patient was the officer's colleague.
He said: "We were called just after 3.10pm this afternoon to reports of a shooting in Southwark Bridge Road.
"We sent two ambulance crews and a duty officer. Two patients were taken to hospital with minor injuries."
Police cordoned off an area of Blackfriars Road from outside Southwark Underground station towards the river.
A number of police vehicles, ambulances and officers were at the scene.
Some people reported that officers had been seen searching along the railway track and a police helicopter had been circling overhead.
Along The Cut at the corner of Hatfields, the junction of the Anchor and Hope pub, officers also cordoned off a small area where a black mountain bike appeared to have been abandoned.
A spokeswoman for Transport for London said: "Blackfriars Road is closed between Union Street and Stamford Street in both directions which involves local diversions for traffic and buses.
"Bus routes that have been diverted are numbers 45, 63, 100 and 344.
"Southwark tube station remains open."
Speaking at the scene, Acting Chief Inspector Malcolm Noone said: "At about 3pm today officers from Kennington police station stopped to speak to a male on a pedal cycle on Coin Street, SE1.
"As the officers approached this male, he dumped the pedal cycle and ran off into Blackfriars Street.
"As the officers ran into Blackfriars Street they were fired upon and the man ran off."
He said two officers had been taken to hospital, one with suspected minor injuries, which may have been a result of his diving out of the way of the shots.
The suspect is believed to be black, the officer said.
"There is an ongoing investigation into the full circumstances and to locate this individual."