Police apprehend a man in a street on the south side of London Bridge in London. AP

Two members of the public were killed when a man wearing a suspected hoax explosive device went on a stabbing rampage before being shot dead on London Bridge, British media said on Friday.

The Press Association domestic news agency and the BBC both quoted unnamed government sources for the toll, but there was no immediate official confirmation from the Metropolitan Police.

Earlier, the Metropolitan Police force said the circumstances were still unclear, but "as a precaution, we are currently responding to this incident as though it is terror-related.”

Scores of police, some armed with submachine guns, rushed to the scene, ushering bemused office workers and tourists out of an area packed with office buildings, banks, restaurants and bars. Workers in office blocks in the area were told to stay inside.

The police force said officers were called just before 2 p.m. "to a stabbing at premises near to London Bridge,” which links the city’s business district with the south bank of the River Thames.

They said a man was detained and "a number of people have been injured.”

Witnesses reported seeing what appeared to be fighting on the bridge and hearing several gunshots. Sky News reported that the man shot was the apparent attacker.

One video posted on social media showed two men struggling on the bridge before police pulled a man in civilian clothes off a black-clad man on the ground. Shots then rang out.

Other images showed police, guns drawn, pointing at a figure on the ground in the distance.

Amanda Hunter said she was on a bus crossing the bridge when she heard shots.

"(The bus) all of a sudden stopped and there was commotion and I looked out the window and I just saw these three police officers going over to a man,” she told the BBC.

"It seemed like there was something in his hand, I'm not 100% sure, but then one of the police officers shot him.”

BBC reporter John McManus was in the area and said he saw figures grappling on the bridge. He said: "I thought it was initially a fight,” but then shots rang out.