Jeremy Corbyn has thanked Labour MPs for taking on new roles in "very difficult circumstances" during the chaotic aftermath of the party's mass shadow cabinet walk-out.
He said having to take part in a leadership contest "wasn't what we wanted to be doing this summer" but said he was determined to fight to ensure the Labour Party moved forward.
Addressing supporters on the day the first ballot papers were sent out, Mr Corbyn said: "This leadership campaign has been brought about because, as you know, there were a series of resignations from the shadow cabinet.
"There was then a series of people who then declined to serve in the shadow cabinet.
"We appointed others to fill their places and I wanted to say a huge thank you to those people who took on completely new jobs.
"Richard Burgon, Becky Long-Bailey, Angela Rayner, Cat Smith... Diane (Abbott) took on a different role in taking on health, and so on.
"And those people have all worked incredibly hard, in very, very difficult circumstances, to get the Labour view across."
Mr Corbyn was speaking to volunteers at a phone bank in Ghousia community centre in Walthamstow, East London, where supporters were making the case for the Labour leader to remain in post.
They applauded as he entered the hall, and posed for group photos and selfies while chatting to the Labour leader.
Mr Corbyn pooh-poohed criticism that he is unelectable by likening himself to the US presidential hopeful Bernie Sanders, who pulled out of the race to back his rival Hillary Clinton.
He said: "We had a message yesterday from Bernie Sanders, saying that they condemned him because he wasn't electable.
"And he said the reason they condemned him was because he was electable. And he represented a threat to the establishment within the USA.
"So I think you can see the parallels that are going on there."
The first ballot papers went out in the morning to the estimated 640,000 members and supporters who will decide whether Mr Corbyn carries on as leader or is replaced by Owen Smith.
With many expected to cast their votes in the coming days, it could be a vital week in the contest - even though polling does not close until September 21.
Promising a new age of politics, Mr Corbyn said of the leadership battle: "We are doing it because we are absolutely determined that there will be no turning back to the days of a Labour Party, fighting election, promising cuts in public expenditure.
"There will be no turning back to abstaining on a welfare reform bill which was taking £12 billion out of the pockets of the poorest and most vulnerable within our society.
"And there will be no return to illegal wars like Iraq," he finished, to rousing applause from the room.