Dominic Cummings says he has no regrets about driving 420km from London to Durham despite the UK lockdown restrictions, saying he believes he behaved "reasonably".
In a highly unusual press conference in the garden of 10 Downing Street, Boris Johnson's chief adviser said he made the journey because of fears over a lack of childcare if he became incapacitated with Covid-19, but also concerns about his family's safety.
Mr Cummings said stories suggested he had opposed lockdown and "did not care about many deaths", but he told reporters: "The truth is that I had argued for lockdown.
"I did not oppose it, but these stories had created a very bad atmosphere around my home.
I was subjected to threats of violence, people came to my house shouting threats, there were posts on social media encouraging attacks."
Mr Cummings said he was worried that "this situation would get worse", and added: "I was worried about the possibility of leaving my wife and child at home all day and often into the night while I worked in Number 10.
"I thought the best thing to do in all the circumstances was to drive to an isolated cottage on my father's farm."
The defence of his actions came amid furious calls for him to resign or be sacked by Mr Johnson for travelling to Durham in March to self-isolate with his family after his wife developed virus symptoms.
He conceded that "reasonable people may well disagree about how I thought about what to do in the circumstances", but said: "I don't regret what I did.
"What I did was actually reasonable in these circumstances. The rules made clear that if you are dealing with small children, that can be exceptional circumstances.
"I was in exceptional circumstances, and the way that I dealt with it was the least risk to everybody concerned if my wife and I had both been unable to look after our four-year-old."
Mr Cummings also said he had not considered resigning and did not offer to do so.
He did not ask the prime minister about his decision and admitted that "arguably, this was a mistake".
He drove north with his wife and son and did not stop on the way, and the next day woke up in pain and "clearly had Covid symptoms", while the prime minister had asked him to publicly give his account, he said.
Labour deputy leader Angela Rayner said the press conference was "painful to watch".
"He clearly broke the rules. The prime minister has failed to act in the national interest. He should have never allowed this situation," she said.