Footage showed police push Mr Tomlinson - video courtesy Guardian.co.uk
The family of a man who died soon after being shoved by police during the G20 protests were "deeply touched" by video of his encounter with officers.
An independent criminal probe was launched after the video emerged showing Ian Tomlinson, 47, being pushed by an officer in London on 1 April.
He died from a heart attack soon after the clash with police.
His stepson Paul King said until the family got all the answers and justice "we can't lay our father to rest".
The Independent Police Complaints Commission (IPCC) has begun a criminal inquiry into the incident and said it would also request a second post mortem test.
It also asked officers involved in the incident to come forward.
Speaking to BBC Radio Five Live Mr King, 26, said the officer seen pushing Mr Tomlinson should be arrested.
"I saw what everyone else is seeing. I have not seen anything different.
"I see the police striking Ian with a baton and violently pushing him to the floor.
"We want answers now. It was only minutes after Ian collapsed from a heart attack. It was clear he was not in any distress or pain when he got up and walked away.
"He clearly had a conversation and asked why he got pushed over by the officers."
Ian Tomlinson lived and worked in the City of London
He added: "I can see my dad, Ian, has his hands in his pockets with his head down walking away, so there was no reason for the officer to push him in the beginning.
"We want answers. We just want answers from anyone that has seen anything."
He urged anybody who has any more footage or photographs of the incident to submit them to the IPCC.
Although Mr King thanked the person who shot the footage he wished the footage had not been in the media first.
"I wish it did not go straight up on the telly because the family's been deeply touched by this.
"My sisters, his daughters, are in a very bad way.
"So is mum and so am I. I am having to be the strong one to talk because Ian is not here to talk."
He said police had not said anything about Mr Tomlinson been assaulted at first but had all along assured the family that they will "get to the bottom of it".
"Hopefully the officer who did push Ian to the ground has been pulled in. We do not know that yet.
"It is very important for the sake of the family here and his kids, we just want justice.
"Until everything comes out and we have the right evidence, we cannot lay our father to rest."