A desperate plea for the whereabouts of a London journalist who was reportedly stabbed in London's Borough Market attack is circulating on social media.

The colleagues and friends of Mr Geoff Ho, who was said to have been stabbed at a pub called Southwark Tavern on Saturday (June 3) night, are posting and spreading photos of him, the Daily Mail Online reported.

"I need to know if Geoff Ho is ok if anyone can help. Stabbed at Southwark Tavern. Last seen in an ambulance," Ms Isabelle Oderberg tweeted early on Sunday morning.

'He called me yesterday and promised to be at my wedding in September. He always keeps his promises," she also wrote. "I just want my friend."

A van swerved into Saturday-night crowds on London Bridge, before three men got out of the vehicle and went on a stabbing rampage through nearby bars in the popular nightlife spot of central London.

The men, who wore hoax suicide vests, were shot dead by police, Metropolitan Police Assistant Commissioner Mark Rowley told reporters.

The attack comes as campaigning for the June 8 election enters its final stages, with the country still traumatised by two other terror attacks this year.

A suicide bomber killed more than 20 people, including children, at a pop concert in Manchester last month. The Islamic State in Iraq and Syria claimed responsibility for that attack.

In March, a man mowed down pedestrians near Parliament before stabbing a police officer.