The alleged terrorist arrested near the Houses of Parliament was "becoming more religious" and his attitudes were "starting to change" during his teens, a school friend of the suspect has told ITV News.

A man, understood be Khalid Mohamed Omar Ali - also known as Khalid Ali, was arrested in Westminster on Thursday on suspicion of preparing an act of terror following a tip-off from a concerned family member.

Markaiu Mason said "his heart sank" when he saw an image of the 27-year-old being pinned to the ground by armed police near a bag full of knives, just metres from Parliament Square.

He remembers Mr Ali as an "amazing person" who was "honest, charismatic and charming" during their time together at Northumberland Park Community School in Tottenham, north London.

But he also claims his friend became increasingly devout when he turned 15.

He said the two would often have "heated discussions" about religion but Mr Ali soon started "preaching more".

"I could see that he was starting to slowly change...his attitudes were starting to change and he was becoming more religious and I thought there's something not quite right here."

Recalling his last conversation with Mr Ali before they lost touch in 2008, Mr Mason said: "I would say, 'You're acting like a crazy person - what are you becoming?' And he would just laugh it off."

Mr Mason said he instantly recognised Mr Ali's face as soon as pictures of his arrest started to circulate.

"My heart sank because he was a really, really great friend and an amazing person at school and to be connected with something like this really did break my heart personally because I know him - well I knew him."