The American city of Philadelphia has called on the National Guard to help keep order after looting and destruction took place in the city on Monday and Tuesday.

Demonstrators have taken to the streets after black American Walter Wallace was shot dead by police.

He was carrying a knife and ignored an order from an officer to drop it.

The protests started peacefully on Monday, but quickly turned grim.

Several business premises have now been damaged, 91 people have been arrested and 30 officers have been injured.

In addition to the army, a larger police force is also deployed on the street.

"Vandalism and looting are not an acceptable form of exercising the right to free speech," said Philadelphia Mayor Jim Kenney.

Protests in the city began hours after a video of 27-year-old Wallace's death was circulated.

Protesters believe Wallace's death is the latest example of excessive police brutality against black Americans.

The Philadelphia rallies follow a wave of protests since George Floyd's death in May.

He died after a cop pushed his knee against his neck for nearly nine minutes.

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Demonstrators and police clash again after black man's death in Philadelphia

Wallace was suffering from bipolar disorder

Wallace was suffering from bipolar disorder, which his wife would have reported to the police.

Images show officers warning him and then drawing their firearms.

Wallace then continues to approach the agents with a knife, after which the agents flinch.

Shortly after, the man was shot seven times.

Wallace's relatives criticize how cops are equipped.

A cousin states that everything would have been very different if officers had had a taser.

"But in this case they had a gun, and that's what they used."