Man stabbed in head in chaotic scene outside London McDonald's

There was "blood everywhere" after a man was stabbed in the head outside a McDonald's restaurant in London's south before citizens pounced on the attacker.

Terrified witnesses watched on as one man allegedly stabbed another man on the busy high street in Bromley full of pedestrians including families with children on Tuesday morning.

The suspect tried to flee the scene after the attack but several "have-a-go heroes" charged him down and held him until police arrived, The Mirror reports.

One woman filmed the chaotic scene following the attack from the across the street as paramedics tended to the calm victim's head and police held the suspect to the ground.

Sirens rang through the air as police cars and ambulances arrived while several witnesses offered help to one woman clearly in shock with blood on her clothes that was likely not hers.

"There's blood everywhere … there's blood on her back," the woman said about the shocked woman.

"And there's a boy over there being held on the floor."

One witness who asked not to be named told the newspaper she was not sure if the woman with blood on her was the victim's partner.

"It was on the pavement, and all over the woman, it was everywhere and I could hear more screaming," the witness said.

“The man who had been stabbed seemed quite calm – or at least he wasn't saying anything."

The woman eventually sat down next to the stabbed man with a bandage around his head while the alleged attacker was arrested and taken to a London police station.

Other instances of 'Have-a-go heroes' have been less violent.

Have-a-go hero' helps out London police by tripping fleeing suspect

Driver threatens cop: 'I should be doing a citizen's arrest'

Woman escapes citizen's arrest with brute strength

Tradies' chase ends in crash

British citizens arrest a man robbing a jewellery store