The referendum on 5 May will be the first across the whole of the UK since 1975 The campaign for a Yes vote in the referendum on changing the way MPs are elected is being officially launched.
Celebrities including comedian Eddie Izzard and former athlete Kriss Akabusi will be at the central London event – being billed as “politician-free”.
On Tuesday Labour leader Ed Miliband and senior Lib Dems urged people to back the “fairer” AV system on 5 May.
The No campaign, launched in February, says AV would mean some people’s votes became worth more than other people’s.
Voters across the UK are being asked whether they want keep the current “first-past-the-post-system” for electing MPs to Westminster or change to the alternative vote (AV) method where candidates are ranked by preference.
The Yes campaign says the alternative vote would ensure MPs secured at least 50% of the votes in their constituency.
At the moment MPs are elected by the first-past-the-post system, where the candidate getting the most votes in a constituency is elected.
On 5 May all registered UK voters will be able to vote Yes or No on whether to change the way MPs are elected to the alternative vote system.
Under the alternative vote system, voters rank candidates in their constituency in order of preference.
Anyone getting more than 50% of first-preference votes is elected.
If no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers’ second choices allocated to those remaining.
This process continues until one candidate has at least 50% of all votes in that round.
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Supporters argue it will provide voters with more choice, force candidates to appeal to a broader section of the public and work harder to get elected.
Those who oppose change say the current system generally leads to stable government and has historically reflected the will of the public in that unpopular governments have been voted out.
They argue it is straightforward and easy to understand, and other systems are more likely to produce indecisive outcomes.
The campaign in favour of change claims it is a people’s movement and has secured the backing of celebrities for the official launch event instead of using politicians.
Messages of support have been sent by broadcaster Stephen Fry and actor John Cleese.
On Tuesday, Mr Miliband shared a platform with leading Lib Dems including former leader Charles Kennedy, party president Tim Farron and Baroness Williams, as well as Green Party leader Caroline Lucas, to urge a Yes vote.
However, Lib Dem leader Nick Clegg was on a trade and diplomatic mission to South America instead.
The Labour leader has urged Mr Clegg to “lie low” during the Yes campaign, given public anger towards the deputy prime minister over student tuition fees and public sector cuts.
Mr Miliband was joined by shadow cabinet colleagues Tessa Jowell and John Denham, but senior Labour figures such as former cabinet members Margaret Beckett and John Prescott are campaigning for a No vote.
At the launch of the No vote, campaign director Matthew Elliott put the total cost of changing to AV at £250m – a figure disputed by supporters of the system.
Under the current Westminster electoral system, voters place a cross next to their preferred candidate. Under the alternative vote they would rank candidates in order of preference.
If no candidate wins more than 50% of the vote the candidate with the fewest votes is eliminated and their backers’ second choices allocated to those remaining, with process continuing until one candidate has more than half the votes in that round.
A separate advertising campaign to raise awareness of the referendum was launched on Friday.
The adverts for television, radio and newspapers will highlight an Electoral Commission information booklet being sent to all households.
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