Something big happened in British politics this summer. After years of being told there was no alternative to austerity and the stale politics of despair, something amazing happened.

Given just half a chance, hundreds of thousands of people seized an opportunity for a new kind of politics. A form of politics that was kinder, more honest and straight talking. One that refused the political counsel of despair and offered a simple yet powerful alternative: Hope.

As we now know it was Jeremy Corbyn that so clearly and forcefully articulated this simple yet powerful message. One that explained a now obvious, self-evident truth - that austerity is a political choice, not a necessity.

A quarter of a million people, 60 per cent of the total leadership vote, overwhelmingly made Jeremy Corbyn their new leader. Shortly afterwards Labour’s conference gave Jeremy and his shadow cabinet a clear mandate to turn the Party into an anti-austerity party of hope and bold alternatives.

Whether people agree with Jeremy Corbyn's politics or not it's becoming increasingly clear his victory has blown politics wide open. As US actor Shia LaBeouf recently exclaimed, “British politics just got very exciting”.

But more important is that these changes are good for our democracy. The British public deserve real choices not forced, technocratic arguments about variations of the same dead end arguments. Once again we've been reminded we should never fear articulating bold, radical and credible alternatives to the problems facing our economy, country and planet.

After years of cuts, privatisation and the handing over of ever more power to unaccountable vested interests, our country is crying out for such new ideas and leadership. On everything from climate change to the housing crisis, we need solutions that are credible, bold and radical. It's why our party recently established an economic advisory group headed by a range of highly respected economists like the Nobel Prize-winning economist Joseph Stiglitz and Thomas Piketty. Yes, we'll be radical but never at the expense of credibility.

So we've broken the mainstream political mould of the past 35 years, offering something new and positive - a kinder way of doing politics. And yet to read some newspapers or listen to some political commentators you could be forgiven for thinking we've ushered in nothing short of political armageddon. A theme many have picked up on is something the Tories are clearly pushing, namely that the “Labour Party is now a threat to our national security, our economic security and your family's security”.

In the unwritten rule of parliamentary politics and etiquette this is, even by Tory standards, a desperate low-blow. If there were any doubt that both they and the powerful vested interests they represent felt threatened, this should put paid to it.

Be under no illusion - the powerful, the exploiters, the excessively wealthy will not pull their punches. By standing up to them, both Jeremy and the Labour Party will face an unparalleled assault that will make what happened to Ed Miliband pale into insignificance.

That's why the need for a social movement to work for a more democratic, equal and decent society inside and outside the Labour Party couldn't be greater. One that can help make the political space necessary for the new ideas we so badly need.The array of vested interests confronting us cannot be taken on by one man or even a single political party in Westminster. Instead, we need a bigger, broader, deeper alliance that can confront such powerful, vested interests.

That's why today, four weeks after ballots closed in the Labour leadership election; I'm so very pleased to announce the launch of Momentum.

Momentum is a grassroots network arising out of, and following on from the Jeremy Corbyn for Labour leader campaign.

It plans are as bold as the challenges that confront it. It will organise in every town, city and village to create a mass movement for real, progressive change.

It will work with Labour members to transform our Party into a democratic institution worthy of its founders' original aspirations. A Party with not just the right policies for a General Election but ultimately the collective will to enact them in government.

But we also understand politics has changed and is changing. The top-down, command and control, monolithic political structures of yesteryear are fast fading. The political eco-system of today is both vast and diverse. Whilst out Party can play a key leadership role in future political change it must also understand it does not have a monopoly on opposing vested interest.

That's why Momentum will strive to bring together progressives campaigning for social, economic and environmental justice across the country. Be they individuals or groups we'll reach out into our communities and workplaces to campaign and organise together on the issues that matter to all of us.

Throughout the campaign, Jeremy spoke about building a social movement to work for a more democratic, equal and decent society. Now is the time to make this a reality and to build on this - Momentum.