All hospitals in England will provide "a truly seven-day NHS" by 2020 under a future Conservative government, David Cameron has said.
At the party's spring forum, Mr Cameron said that more hospitals must provide top-level treatment at the weekend, starting with emergency care.
In a wide-ranging speech, he said his party's message to various sections of the population was: "We're with you."
Labour said Tory plans for "extreme" spending cuts threatened the NHS.
It has put the health service at the forefront of its own election campaign, with leader Ed Miliband promising on Friday to cap the amount of profit private firms can make from the NHS in England.
By Ben Wright, BBC political correspondent
This spring forum is about fighting back against Labour on the NHS.
Polls show a clear, large consistent lead for Ed Miliband's party on the NHS. It is the foundation stone of Labour's campaign.
But the Conservatives are desperate to chip away at it with promises of protected real-terms increases in funding and an extra £2bn a year.
The Conservatives are committing to providing full weekend hospital care in England - [Only registered and activated users can see links. ].
The big question of course is, "Where will the money come from?", but this is about politics.
This is an attempt to try to neutralise the NHS and enable the Conservatives to return to what they want to be talking about - the economy.
Speaking at the forum in Manchester, Mr Cameron warned that figures showed patients were "more likely to die" if they were admitted at weekends.
According to the Conservatives, official studies suggest mortality rates for those admitted on Saturdays and Sundays are 11% and 16% higher respectively than for those admitted on Wednesdays.
This is at best an empty pledge and at worst shameless political game-playingDr Mark Porter, British Medical Association chairman"For years it's been too hard to access the NHS out of hours. But illness doesn't respect working hours. Heart attacks, major accidents, babies - these things don't just come from nine to five," Mr Cameron said.
At weekends, he said, "some of the resources are not up and running. The key decision-makers aren't always there.
"With a future Conservative government, we would have a truly seven-day NHS.
"Already millions more people can see a GP seven days a week but by 2020 I want this for everyone, with hospitals properly staffed especially for urgent and emergency care, so that everyone will have access to the NHS services they need seven days a week by 2020 - the first country in the world to make this happen."
During his speech, Mr Cameron also reiterated a number of election campaign pledges, including:
- No tax on the first £12,500 of earnings
- No 40p rate until earnings reach £50,000
Andy Burnham, Labour's shadow health secretary, said the plans were not credible without investment in extra NHS staff.
"With the NHS in increasing financial distress, David Cameron must set out clearly how it will be paid for," he said.
The Liberal Democrats said NHS England already had plans to open hospitals and GP surgeries seven days a week, while UKIP said the Tories had "degenerated the NHS beyond all recognition" during the last five years in government.
Dr Mark Porter, chairman of the British Medical Association, said that the Conservatives' funding commitment was not even enough to maintain existing services, and that a funding gap of up to £30bn a year was predicted to open up.
He said: "Without a detailed, fully-costed plan to provide the staff and resources needed to deliver more seven-day services, this is at best an empty pledge and at worst shameless political game-playing with the NHS ahead of the election."
The Conservatives have pledged to guarantee a real-term increase in funding for the NHS during the next Parliament, extending the ring-fence in place for the past five years. Labour has said it will spend £2.5bn more than its opponents.
Health Secretary Jeremy Hunt said government reforms of the NHS were saving £1.5bn a year but that the NHS "will need more money".
He added that the NHS's own sums suggested the predicted £30bn annual shortfall could be "reduced with efficiency changes, and we're backing that plan".
New clinical standards set out in 2013 require hospitals to provide seven-day access to diagnostic tests, such as X-rays, ultrasound, MRI scans and pathology, as well as providing access to multi-disciplinary teams, which include expert nurses and physiotherapists.
In its blueprint for services over the next five years, published last October, NHS England said hospital patients should have access to seven-day services by 2020 - "where this makes a clinical difference to outcomes".