Jeremy Corbyn is expected to address a crowd of hundreds at a protest event in Manchester during the Conservative Party conference.
Leaders of the main parties traditionally stay away from rivals' conferences, but Mr Corbyn will appear at a public meeting organised by the Communication Workers' Union.
The Labour leader's visit to Manchester comes after tens of thousands of activists marched through the city centre to protest about Government policies on the conference's opening day.
Although the march, involving an estimated 60,000 people, was largely peaceful, there were some ugly scenes and four arrests.
Mr Corbyn had appealed for activists to "focus on policy and take no part in personal attacks".
But some elements of the protest ignored his call for "kinder" politics and spat at conference-goers, including journalists, while a young Tory was hit with an egg.
A spokesman for Mr Corbyn said he thought the abuse suffered by reporters was "inexcusable" and "journalists must be able to do their jobs".
Green leader Natalie Bennett and singer turned activist Charlotte Church are also expected at today's event in Manchester Cathedral.
The CWU's campaign sets out to protect postal services and decent employment standards in the postal industry, which the union claims is under threat from privatisation and aggressive regulation.
Ahead of the event CWU general secretary Dave Ward said: "The level of interest in this event clearly shows that a movement is building in our country that calls for a different kind of politics. The CWU is central to this movement, as is protecting the UK's postal services."