The Liberal Democrats kept up the rhetorical pressure on the Conservatives when party president Tim Farron described Thatcherism as "organised wickedness" and claimed David Cameron's lasting legacy as prime minister would be his decision to defend an "indefensible" electoral system.

At a cross-party event organised by the yes campaign in the referendum on the alternative vote system, Farron said Cameron had turned his back on a long Conservative tradition, stretching back to Disraeli and Baldwin, in favour of evolution of the voting system.

He also claimed first past the post had made possible the "organised wickedness" of unemployment under years of Thatcherism.

His criticism of former prime minister Margaret Thatcher prompted a rebuke from Nigel Farage, the leader of the UK Independence party, who called on all sides in the referendum campaign not to resort to personal abuse.

Farron said: "The more shrill and terrified the establishment sound, the more you know that voting yes is right.

"I thought he [Cameron] stood for change, progress and reform – he was the future once.

"Does he really want to be remembered as the last defender of a discredited system? Whether he wins or loses, his legacy will be defending the indefensible."

Farron said the mass unemployment of the 1980s "was organised wickedness given the veneer of legitimacy by an electoral system that gave the Conservatives 100% of Britain's power despite being opposed by 60% of the electorate".

In an appeal to the many confused, indifferent and wavering voters, he said: "Do we want to be the generation mocked by our children and grandchildren for bottling this change, or do we want stand up to the press barons in this most British of weeks to make a very British change, a small change that will make a big difference?"

Farage and Farron were sharing the yes platform with former Labour home secretary Alan Johnson and the leader of the Green party, Caroline Lucas.

The yes event came as campaign organisers said the outcome of the referendum would turn on whether Labour voters decided to back change and saw the status quo of first past the post as likely to entrench the Tory party in power.

Johnson acknowledged that Labour was divided over the issue – the natural result of the fact that the party can win under first past the post.

The Labour no campaign has for weeks claimed a majority of the parliamentary party. Ed Miliband has said Labour MPs are free to take whatever position they want.

Johnson said he believed in their hearts most Labour activists supported change, implying that they were being held back by their contempt for the Liberal Democrat leadership.

"I cannot imagine anyone joining a radical, progressive party like the Labour party and thinking the electoral system is absolutely perfect and does not need change in any way," he said.

He argued that "even though we can win through first past the post, we think it is a miserably disempowering system that belongs in the past. It is not of this age."

Johnson said the current system had been devised by a tiny political elite when most working men and women did not have the vote, and had been designed to suit the establishment's preference for a two party system.

Labour should not become part of that establishment, he said, adding: "It is against our history and our principles."

Farage, a supporter of a proportional system, complained: "The second-ever national referendum has degenerated into a war of abuse. Professional politicians bickering with one another does nothing to enthuse people to go out and vote next Thursday."

He said the people to whom he had spoken were completely turned off by the referendum, adding: "Very bad mistakes have been made on both sides."