Tory leader Michael Howard has accused Tony Blair of "pussyfooting" around immigration problems as the election campaign resumed after a two-day pause.
Kennedy: Remember positive impact of immigrants
Mr Howard said immigration was out of control and said the issue "should not be swept under the carpet".
But Labour Cabinet minister Peter Hain accused the Conservatives of "scurrilous, right-wing, ugly tactics".
Lib Dem leader Charles Kennedy said Britain benefited from being a multi-racial, multi-ethnic society.
Michael Howard makes immigration speech in Telford, Shropshire
Tony Blair speaks in the North East, Gordon Brown in Shipley, Yorkshire
Charles Kennedy unveils "battle bus" in London
UKIP conference in Perth, Scotland
Campaigning was suspended on Friday for the Pope's funeral and remained low-key on Saturday while party leaders attended the royal wedding celebrations.
The truce is over and three opinion polls in Sunday's newspapers suggest Labour has a lead of between 2% and 7% over the Tories, with the Lib Dems another 14% behind.
In a speech in Telford, Mr Howard went on the offensive over immigration denying it was racist to broach the subject.
He said that failing to speak up would only help the "bigots who preach racial hatred and the people smugglers who profit from other people's misery".
The Tory leader said he believed "the current system is deeply inhumane and profoundly unjust".
He continued: "Mr Blair may want to pussyfoot around this issue, but I don't. The majority of British people, whatever their background, are united on this issue. They want immigration to be limited and controlled
"For too many years immigration has been a no-go area for political debate.
"It's not racist to talk about immigration. It's not racist to criticise the system. It's not racist to want to limit the numbers. It's just plain common sense."
The Tory leader stressed the UK had benefited from immigration economically and culturally.
Mr Kennedy meanwhile, speaking on the BBC's Breakfast with Frost, urged fellow politicians to focus on hope and not fear. He said there should be an independent body to decide the level of immigration into the UK.
His speech comes a day after the UN's refugee agency urged politicians to act responsibly and not spread "asylum myths".
Tony Blair preferred to focus on education during a speech in Sedgefield - contrasting his record with that of the previous Conservative government.
But Mr Hain, Labour's leader of the House of Commons, responded to the Tory attack on immigration, telling GMTV that Labour understood it was an issue people were concerned by.
"We have been seeking to tackle it. Michael Howard is just shamelessly using this issue to try and scare people back into voting Conservative without proposing any workable solution," he said.
Labour also attacked the detail of Conservative plans for processing all asylum claims overseas and putting annual limits on immigration and asylum.
It highlighted the views of ex-Tory immigration minister Charles Wardle, who worked with Mr Howard at the Home Office.
Mr Wardle, who lost the Tory whip in 2001 for supporting an independent candidate, says he is not a member of any political party.
But he says Labour has the most practical immigration policies and argues that Mr Howard's proposals show he is "utterly unsuited" to be prime minister.
The Lib Dems say they are determined to be positive during the election - a message Mr Kennedy stressed in a speech to candidates and activists in London.
The main theme for his speech was how to make the UK a better place to live. He said he would press for fairness and social justice through better pensions, replacing the council tax and scrapping university tuition fees.
Among the smaller parties, UK Independence Party leader Roger Knapman attacked Tory policy on immigration as "so unworkable it was almost laughable".
"Mr Howard's problem is that not only could he not implement it without first leaving the EU, but his own MEPs voted in favour of EU enlargement, which brought 133,000 immigrants."