The Old Kent Road was closed southbound near the Bricklayers Arms roundabout on Monday morning after a man was found with head injuries in the early hours.
A 36-year-old man suffering from head injuries was taken to a south London hospital where his condition is described as serious.
Officers from Southwark CID are keeping an open mind as to how the man was injured.
Old Kent Road was closed southbound until at least 12 noon while police investigated the incident.
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