An ex-girlfriend of Oscar Pistorius testified this morning that the Paralympic and Olympic sprinter often carried a gun with him and once fired at a traffic light after a confrontation with police.

Samantha Taylor, who met Mr Pistorius in 2010 when she was 17, described the sprinter as someone familiar with guns, prone to fits of anger and an unfaithful boyfriend.

Ms Taylor took to the stand, testifying that in 2010 on the way back from a day trip to a popular city getaway Mr Pistorius was in a car that was pulled over by the police for speeding.

When a police officer inspected Mr Pistorius's weapon, left on a seat when he exited the car, Mr Pistorius became angry, she said.

"Oscar shouted at the policeman and said he shouldn't touch his gun," she said. 

He later shot at a traffic light, "out of spite after being stopped".

"He was angry at the police after being stopped, thereafter when they wanted to fire a shot they found it funny," she said, referring to Mr Pistorius and a friend who was with him at the time.

The firing of the weapon is one of three firearms charges Mr Pistorius faces in addition to a charge of murdering his girlfriend Reeva Steenkamp on St Valentine's Day 2013.

Frequently breaking down in tears, Ms Taylor recounted the history of the pair's troubled relationship.

She said they met at a rugby match and later "he added me on Facebook".

Their relationship ended in 2011, Ms Taylor claimed, because "he cheated on me with Reeva Steenkamp".

Ms Taylor also dismissed the defence's assertion that witnesses heard Mr Pistorius and not Ms Steenkamp scream on that 14 February.

"That is not true, he sounds like a man" when he screams, she said.