" We must review, redouble and intensify our efforts to provide an alternative government "
Mr Hague, who has already congratulated Tony Blair on his victory, said at his count in Richmond, North Yorkshire that there was much for the Conservative Party to reflect upon.
The Tories won 166 seats - one more than in 1997 - throwing both the party's campaign and William Hague's leadership into doubt.
Mr Hague said: "Clearly, we in our party must review, redouble and intensify our efforts to provide an alternative government for the country in the future."
Two senior party figures have already criticised the Tory election campaign.
Pro-European backbencher Ian Taylor said the Conservatives had used the wrong strategy.
Mr Taylor said Mr Hague would have to "make his own mind up" about his future as party leader.
Shrill and extremist
And earlier former Conservative Home Secretary Lord Brittan told BBC News: "They made a huge strategic misjudgement in placing so much emphasis on Europe and asylum seekers.
"That did give the impression of a party that was shrill and extremist."
He said a leadership contest would be a distraction about personalities when what was needed was a "radical" look at policy and presentation.
Senior party figures, including shadow home secretary Ann Widdecombe and party chairman Michael Ancram, paid tribute to Mr Hague's leadership.
Miss Widdecombe said: "What we have got to do is move on and make sure we can win next time. We don't want to spend the next five years contemplating our navels."
The favourite to succeed Mr Hague - Michael Portillo also towed a loyal line, saying as the early results arrived that he should remain leader whatever the result.
After holding his Kensington and Chelsea seat, shadow chancellor Mr Portillo said: "It should lead us to a period of reflection. I hope that nobody will do anything hasty in the coming hours and days which they might regret thereafter."
Former Tory chancellor Ken Clarke was another party figure who thought Mr Hague would keep his job.
The BBC's exit poll suggested former Tory voters most want Mr Clarke leading the party.
But he told BBC News: "I am a pro-European which would make the party unlikely to turn to me at the present time and I'm not going beyond that."
So far the Conservatives have achieved a net gain of only one seat.
An early blow to the party was defeat in their key target seat Torbay, Devon.
Lib Dem gains
The Lib Dems went on to win Cheadle, Guildford and Teignbridge, as well as Mid Dorset and Poole, from the Tories.
In some of those seats it is thought the Tories lost votes to the UK Independence Party.
But the Tories gained a measure of revenge through victory in Taunton, where Lib Dem Jackie Ballard lost her seat.
Mr Bell moved to fight another seat, Brentwood and Ongar. But, despite a strong showing, he was beaten by the Tory, Eric Pickles.
The Tory gain of Upminster from the Labour Party was another rare scene of Conservative celebration.
Back on the map
Hopes of a full-scale Tory revival were dashed in Scotland, where the party is set to come fourth in terms of votes.
But the Conservatives did win Galloway and Upper Nithsdale - putting the Conservatives back on the map north of the border.
Scottish Tory leader David McLetchie said: "It just shows that reports of our death are greatly exaggerated."
But Mr McLetchie said he was "deeply disappointed" for former Tory Foreign Secretary Sir Malcolm Rifkind, who failed in his attempt to regain the Edinburgh Pentlands seat he lost in 1997.
The Conservatives again did not win a single seat in Wales.