The police officer who pushed Ian Tomlinson to the ground during the G20 protests two years ago told an inquest into the newspaper seller's death that he had been "in fear for my life".

PC Simon Harwood was advised by the coroner, Peter Thornton, that he was not obliged to answer any question that could incriminate him as he took the witness stand for the first time and came face to face with Tomlinson's grieving family.

His opening remark that he was "here to help the family" was met with snorts of derision in the public gallery and Tomlinson's son, Paul King, stormed out of the hearing. Tomlinson's widow, Julia, also left the room, followed by several tearful members of the family.

Thornton told Harwood: "I'm sure that you know, and no doubt will have been advised, that you are not obliged to answer any question tending to incriminate you. It may well be that I shall repeat that warning to you later."

Harwood, dressed in a grey suit and patterned yellow tie, said: "I'm very aware of that. I'm here as a witness to help with the inquest and also to give some sort of answers to help the family. So I'm here to answer questions, just to help."

There were scuffles outside the International Dispute Resolution Centre in Fleet Street when Harwood arrived at the inquest. Security staff escorted him past waiting photographers.

Harwood's evidence to the inquest is likely to last at least two days and will play a crucial role in deciding the circumstances of Tomlinson's death almost exactly two years ago. Tomlinson staggered a hundred yards from where the police officer pushed him and died within minutes of the incident.

The officer was questioned by Alison Hewitt, counsel for the inquest on the events leading up to the encounter with Tomlinson in Royal Exchange Passage in the City of London on 1 April 2009.

Harwood, a police carrier driver on the day, described how he saw a demonstrator crouching at the rear of one of the police vehicles and drawing or writing something on the bodywork.

He told the inquest: "I then made my way towards the person concerned". His intention, he said, was to arrest the man. Asked if he believed an arrest was the right course of action, he said it was his "duty" because the "blatant disregard" the protester showed him gave him no other choice. "I felt it was done right in front of me, so close to my position, I believed it was my duty to go and take some course of action towards this male."

But the arrest caused other protesters to react as the arrest began going awry and his target struggled to break free. He said he had become the centre of attention, with the protesters jeering and shouting.

There was a "large gasp" from the crowd as the suspect he was holding collided with the door of a police van. Footage showed him leading the man further away from the vans with the crowd surging behind him.

He said: "At the time, because he was becoming more aggressive, more hostile, I was starting to believe that this was getting out of control. I was aware there was a very hostile crowd and I was actually in fear for my life then from what was coming towards me."

Members of Tomlinson's family shook their heads when they heard the officer say he was in fear for his life.

Harwood said that before going out on duty that day he had made sure his police numbers were displayed on his uniform.

The jury also heard Harwood offer some explanation about his uniform. Jurors have already been shown pictures of Harwood's distinctive dress, which marked him apart from other officers. He was wearing a yellow jacket tucked into his trousers and a balaclava covering the lower half of his face. Photographs also appear to suggest his badge numbers had not been showing.

Harwood said he had secured his epaulettes to the shoulders of his jacket with pins. "I always have my numbers [visible] as it is a policy, a Metropolitan police policy, to always have your numbers displayed, especially on public order."

The officer added that his jacket was tucked into his trousers "for health and safety reasons", enabling him to access the weapons and tools on his belt.

The balaclava, he said, was lifted as a precaution to protect him from injury.

He said of the policing of the demonstration: "There was a general feeling that it would be robust policing, not just going and picking on people, but robust as in keeping the demonstration where it should be."

The inquest continues.