The incident unfolded ahead of the planned closure of the bridge for a minute's silence at 2:40pm and a vigil commemorating those killed last week.
Two ambulances, police cars and fire engines were called to the bridge while boats were launched on the River Thames.
Witness Halle George tweeted: 'Jesus Christ, someone just jumped off the #westminsterbridge'
John Kindervag added: 'Crossing Westminster Bridge in London. Police and ambulances everywhere. Looks like somebody went over the bridge into the water.'
Another Twitter user wrote: 'Come out of hospital to Westminster Bridge and someone has just jumped off the bridge a week after attack there.'
The bridge is due to host a ceremony to remember the victim's of last week's attack this afternoon.
Police officers and members of the public are due to hold hands and form chains on the pavement where more than 50 people were injured and three were killed.
From 2:15pm, around 500 faith leaders will walk across the bridge before a minute's silence is held.
Four people were killed and dozens more injured in the 82-second assault last Wednesday.
Kurt Cochran, 54, Leslie Rhodes, 75, and Aysha Frade, 44, died after Muslim convert Masood drove at pedestrians on Westminster Bridge.
The 52-year-old terrorist was shot dead by armed police after fatally knifing Pc Keith Palmer, 48, in the Palace of Westminster's cobbled forecourt.
An inquest for the victims will open and be adjourned at Westminster's Coroner's Court this afternoon.
/Daily mail/
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