The Tory leader said at 63 he could hang up his boots and enjoy his retirement but he wanted to "battle for Britain", the country he loved.
Mr Howard urged people to read the Tory manifesto and "see how we've changed". He warned that re-electing Labour would mean "five more years of smirking".
Earlier Mr Blair attacked the "flawed" Tory plans for the economy.
CONSERVATIVE MANIFESTO Read the Conservative manifesto in full (709KB) Most computers will open PDF documents automatically, but you may need to download Adobe Acrobat Reader. Download and install the reader here
Lib Dem leader Charles Kennedy had to cancel plans to campaign in Devon to be with heavily pregnant wife Sarah, who has been taken into hospital.
His party have now postponed their manifesto launch which had been planned for Tuesday and deputy leader Sir Menzies Campbell is taking over his engagements.
Mr Howard, the first of the main party leaders to launch his election manifesto, said: "It is time for change, it is time for action."
He said the manifesto was purposely thin so people would actually read it.
"You don't have to settle for second best - your vote can change things," said Mr Howard.
KEY MANIFESTO PLEDGES Matrons put in charge of fighting hospital superbugs Headteachers have final say on school expulsions Quotas for immigration and asylum 40,000 more police
Analysis: Tory manifesto Fact check: Tory budgets
"If you long for cleaner hospitals, more police, school discipline, controlled immigration, lower taxes and accountability - you can vote for it, on 5 May.
"On 5 May you can bring an end to the years of let-down, and start sorting out the things that matter.
"On 5 May you can let the sunshine of hope break through the clouds of disappointment we all feel."
The Tory leader went on from the launch in London to unveil policies for Scotland and Wales in Glasgow and Cardiff.
Official campaign
Labour also unveiled part of their manifesto, focusing on their plans for education and a six point pledge card for the economy.
HAVE YOUR SAY In my view, immigration and the economy - especially pensions - are the two main election issues
Paul P, Birmingham, UK
Send us your comments Voter's panel views
The Lib Dems also outlined their plans to recruit 21,000 more teachers to provide smaller primary school classes.
Monday was the first official day of campaigning as the Queen formally dissolved Parliament - meaning there are no longer any MPs, although ministers retain their government roles.
The Tories' manifesto does not include the further details of plans for tax cuts. Those will be held back until later in the campaign.
In the manifesto, Mr Howard promises voters greater control over their money and greater choice in public services. He says the Tories offer "action" not Labour's talk.
This is a new manifesto from the same old Tories
Alan Milburn
Full text of Howard speech Kennedy's teachers pledge
Mr Blair and Gordon Brown are continuing their joint campaigning and fronted Labour's first election rally on Monday evening, as part of a two-day national tour.
At the event in Oldham, Mr Blair said the Tory manifesto was not just "threadbare" but was based on fear rather than hope.
Earlier, Labour's election co-ordinator Alan Milburn called the Tory manifesto a "fraudulent prospectus" that was "full of holes".
"This is a new manifesto from the same old Tories," he said. "It confirms they still stand for privilege not opportunity in our country."
For the Lib Dems, Mr Kennedy said: "People have already decided that they do not want the Conservatives back running health and education.
"They also know that Conservative pledges to put money back into public services at the same time as cutting taxes and borrowing are simply not credible."
The Conservative Party of the United Kingdom, Monday, launched its slimline manifesto for the May 5th general election, a twenty-seven page document entitled ''The British Dream''.

The manifesto focuses on five key areas:

'''Lower tax and value for money''': The party plans to make up to £12 billion of savings annually by reducing beurocracy such as quangos.  £8 billion will reduce previous deficit, and £4 billion of tax cuts will be made, especially to what have been called "stealth taxes".  The party promised that funding for education, health, transport and international development would not be cut, and spending on police, defence and pensions would be increased.

'''Flexible childcare and school discipline''': The manifesto promised an increase in maternity pay and more choice of childcare.  Under a conservative government more independence over expulsions and admissions would be given to school heads and governors.  Special schools for disruptive pupils would be created, and more vocational courses would be created for 14-16 year olds.

'''Better healthcare and cleaner hospitals''': A major feature of the coservative campaign has been hospital sourced infections, and in respose the party have promised to introduce ward matrons charged with keeping hospitals clean, with the authority to close wards with MRSA infections.  A Conservative government would contribute funding towards operations in private hospitals.  Econimic migrants with HIV or TB would not be allowed to live or work in Britain.

'''Safer communities and more police''': A Conservative government would increase police recruitment by 5,000 a year.  The manifesto promised to increase prison terms and increase Britain's prison capacity by 20,000 places.  The party would reverse the Labour government's decision to relax laws prohibiting Canabis use.

'''Secure borders and controlled immigration''': The manifesto proposes a new border police at Britain's busiest air and sea ports, with 24 hour surveilance.  The party would set a quota on economic migrants and reject asylym-seekers who are not vetted by the UNHCR.

Party leader Michael Howard summed up the manifesto: "If you long for cleaner hospitals, more police, school discipline, controlled immigration, lower taxes and accountability - you can vote for it, on 5 May", accusing prime-minister Tony Blair, who is campaigning for his third term, of letting the country down.