Cameron, Ashdown and Kinnock in phone campaign
Party leaders past and present joined forces to attempt to persuade people to back the campaign to remain in the EU.
Prime Minister David Cameron was flanked by ex-Lib Dem leader Paddy Ashdown and Labour's Neil Kinnock for the telephone campaigning session.
It comes ahead of the first official day of the EU campaign, which brings in new spending limits for both sides.
EU exit campaigners have criticised the amount already spent by the government promoting its case.
They say the £9m government pro-EU leaflet campaign exceeds the £7m the Out campaign is allowed to spend during the official campaign, and have accused ministers of misusing public money on "one-sided propaganda".
But the government says it is "not neutral" in the referendum and has a duty to set out its case and provide voters with the "facts".
Manning the phones at the Britain Stronger In Europe headquarters with Lord Kinnock and Lord Ashdown, Mr Cameron said there was a "great team" getting behind the In campaign.
The prime minister said that while he and his political rivals disagreed on a lot "we all come together to support the idea of Britain staying in a reformed EU".
Lord Kinnock said he thought it was "very good" taking part in a multi-party event because "it underlines this issue goes way beyond the usual boundaries of partisan politics".
The campaign push came a day ahead of the start of the official referendum campaign period, when restrictions on campaign spending will come in to play.
Stronger In and Vote Leave - the official Remain and Leave campaigns - will be allowed to spend up to £7m each, with other campaign groups restricted to £700,000.
While the internet and social media have opened up new methods of campaigning, the more traditional methods - such as leafleting and phone canvassing - still play a vital role.
Earlier on in the campaign trail, Leave campaigners hit the phones to encourage voters to back an EU exit.
The line-up at the Vote Leave launch included several cabinet ministers including Iain Duncan Smith - who has since resigned as work and pensions secretary - Northern Ireland Secretary Theresa Villiers and Commons leader Chris Grayling.
Meanwhile, London mayor Boris Johnson - one of the high profile figures backing a vote to leave - opted for a more eye-catching large red lorry to spread the Vote Leave message.