Jeremy Corbyn says Labour's huge membership has created a "social movement" that will help the party return to government in 2020.
The leader told activists in Salford that members "want to see a different world and do things very differently".
He also condemned abuse among members, saying "it has no place in our party".
It comes as more than 40 female Labour MPs have written to Mr Corbyn saying he has failed to do enough to prevent "disgusting" threats against members.
Mr Corbyn said: "We are a social movement and we will only win the next general election because we are that movement of people all around the country who want to see a different world and do things very differently."
He added that "some people say that isn't how politics is done, and that it is solely what happens in parliament that is important", but he insisted "changes come because people want those changes to come and Parliament has to be influenced in the way those changes come about".
Meanwhile, leadership rival Owen Smith has promised key party roles to women.
The Pontypridd MP vowed to act on gender inequality - promising two of his four most senior cabinet roles would go to women if he became prime minister.
Mr Smith said his wife, Liz, had been a victim of online abuse and claimed there was now a level of abuse, anti-semitism and misogyny in Labour that was not there before Mr Corbyn became leader.
"My wife stood recently for a community councillor position in the village where we live in South Wales and was subject to a torrent of online abuse. It's a community council position."
He added: "I think it's just absolutely unacceptable."
A group of female MPs - including former shadow ministers Heidi Alexander, Paula Sherriff and Kerry McCarthy - have written to Mr Corbyn saying intimidation was being carried out "in your name".
"Rape threats, death threats, smashed cars and bricks through windows are disgusting and totally unacceptable in any situation," the letter said.
Disagreement in party
But Mr Corbyn said: "I don't do personal abuse, I don't respond to personal abuse, I condemn any abuse from others. It has no place in our party."
He added: "I know some people are angry at the actions of some MPs but where we have disagreement in the Labour Party we settle it through democratic means - not coups, not intimidation, and not abuse."
Simultaneous launch events were due to take place in cities across the country, including Hull, Newcastle, Cambridge, Bristol, Nottingham, Stoke, London and Cardiff.
MI5's 'dark practices'
On Friday, Unite trade union leader Len McCluskey suggested in an interview with the Guardian that the security services could be behind the abuse and intimidation of MPs on social media.
Mr McCluskey said MI5 could be using "dark practices" to "stir up trouble" for Mr Corbyn, arguing that spies had infiltrated trade unions in the past, and that the truth about it had been suppressed for 30 years under the rule on keeping classified documents out of the public domain.
Mr Smith distanced himself from the suggestion, saying at a campaign rally in Manchester: "I'm not sure that's entirely right."
Mr Corbyn is facing a challenge from former shadow work and pensions secretary Mr Smith, after Labour MPs overwhelmingly backed a motion of no-confidence in their leader.
The winner of the leadership contest will be announced on 24 September.