The royal audience — possibly Cameron's last as prime minister — came as Britain's Parliament was officially dissolved ahead of the May 7 vote.
Polls, bookmakers and politics-watchers say the election is too close to call, and no party is expected to win a majority of seats in the House of Commons.
Some form of coalition government is likely, and smaller parties — such as the Scottish and Welsh nationalists, the Greens and the anti-Europeans — could hold the balance of power.
"This is the most unpredictable election we have seen in our lifetimes," said Nigel Farage, leader of the anti-Europe U.K. Independence Party, which is currently running third in popular support. "All bets are off."
While issues such as the European Union and immigration will play a big role in the campaign, both Cameron's Conservatives and their main opposition, the Labour Party, are focusing their pitches on the economy.
Cameron said a Labour victory would bring "economic chaos" and threaten Britain's recovery from the Great Recession.
"Debt will rise and jobs will be lost as a result," he said.
Speaking outside his 10 Downing St. office after meeting the queen at Buckingham Palace, Cameron said when he took office in 2010, "Britain was on the brink."
Now, he said, "Britain is back on her feet again," and growing faster than other G-7 economies.
But Labour leader Ed Miliband argued that for many voters, that recovery "feels like it's happening to someone else, somewhere else." He kicked off campaigning with a speech aimed at reassuring business that Labour won't increase tax and red tape.
And he called the Conservatives' vow to hold a referendum on whether Britain should leave the 28-nation EU a "clear and present danger" to British businesses.
Britain's electoral system means only Labour or the Conservatives, as the country's two biggest parties, can hope to lead the next government.
But voters are defecting in droves to alternatives, including the pro-independence Scottish National Party and UKIP, which wants to leave the EU and impose tough controls on immigration.
UKIP, which has just two lawmakers of 650 in the House of Commons, launched its campaign with a photo-call near Parliament. It unveiled five election promises, including exiting the EU and cutting foreign aid spending.
Farage called foreign aid "a waste of money" and said the funds would be used instead to cut the deficit and strengthen the armed forces.
Cameron's visit to the palace was a courtesy, since this election ends the historic practice of prime ministers asking the monarch to dissolve Parliament. That is now done automatically. The same law set election dates to be the first Thursday in May every five years, unless the government loses a confidence vote in Parliament.
Associated Press writer Gregory Katz contributed to this report.