Mr Hague said he did not know where the Libyan leader was, but he had been "clearly rejected" by his own people.
While opposition forces control large parts of Tripoli, there are reports of gun battles with regime loyalists.
The UK and France have invited Libya's opposition political authority to an international conference next week.
Britain is also talking to humanitarian agencies about how to get emergency medical supplies into Tripoli.
Aid groups have warned that the city's hospitals and medical centres have been "overwhelmed with casualties" and that medicine and fuel supplies are running low.
The United Nations' envoy in Tripoli has said that opposition forces now control the vast majority of the capital, but there are reports of pockets of resistance from pro-Gaddafi forces.
Rebel forces say they expect to detain Col Gaddafi soon, but he has urged forces loyal to him to continue fighting to "cleanse the city".
After chairing a meeting of the National Security Council meeting - the third to be held in consecutive days over Libya - Mr Hague urged Col Gaddafi to stop making "delusional statements".Battle for Tripoli
He said Col Gaddafi should accept the fact that he had "lost control" of most of Tripoli and large parts of Libya and would not be able to regain it.
"There is a clear, fundamental, decisive rejection of that regime by the people of Libya," he said.Continue reading the main story
The National Transitional Council faces an important and significant challenge in translating a popular uprising into stable government for Libya”End Quote Douglas Alexander Shadow foreign secretary "He [Col Gaddafi] should be telling his dwindling and remaining forces now to stand down."
He said the government remained "actively engaged" in both military and humanitarian efforts in Libya as well as United Nations moves to unfreeze £12bn Libyan assets in the UK held withheld since the conflict began.
Downing Street said David Cameron and French President Nicolas Sarkozy had invited Libya's National Transitional Council (NTC) to attend a conference on the country's future in Paris next week.
"This will be an opportunity for the NTC to set out how the international community can help them on the path to establishing a free, democratic and inclusive Libya and for all those who wish to support Libya to discuss the role that they can play to support this," a Number 10 spokesman said.British nationals
As fighting continued in Tripoli, BBC journalists confined for days in a Tripoli hotel by armed guards loyal to the Gaddafi regime were released after representatives of the International Committee of the Red Cross arranged for them to leave.
Those freed from the Hotel Rixos also included a US congressman and an Indian MP.
But amid concerns for the safety of the small number of British nationals still in Tripoli, it emerged that a Maltese vessel which the Foreign Office suggested that Britons could use to leave the capital had not reached its intended destination, instead returning to Malta.
The Maltese authorities said it was not considered "safe enough" to transfer passengers from the city centre to the port.
The UK government has advised all Britons still in Libya to leave the country immediately.
Security experts said they expected there to be continued attacks from remnants of the regime over the coming days and weeks amid fears of reprisals should pro-Gaddafi forces lay down their arms.'Significant challenges' Demonstrators outside the Libyan Embassy in London wore wristbands showing the date the uprising began
For Labour, shadow foreign secretary Douglas Alexander said it was clear the days of Gaddafi rule were over and that he was "on the run".
But he said there was a lot of work to be done to bring stability to the country, to disarm combatants and to ensure the Libyan people could get access to basic amenities.
"Taking a city is not the same as running a city," he told the BBC.
"The National Transitional Council faces an important and significant challenge in translating a popular uprising into stable government for Libya."
Mr Alexander said the UK could play a role in supporting what he said must be a Libyan-led process in developing new constitutional arrangements for the country.Military operations
Nato has said its military operations will continue until all pro-Gaddafi forces surrender.
The UK has taken a lead role in five months of strikes against the apparatus of the Gaddafi regime, in accordance with a UN mandate to protect Libyan civilians.
The UK has also been providing logistical advice and training to the anti-Gaddafi forces but has effectively ruled out sending in ground troops as part of a transition to a new government.
The only Conservative MP to vote against military intervention in Libya has continued to question the motives behind the campaign.
John Baron described the action as the "longest assassination attempt in history" and said he remained concerned that successors to the Gaddafi regime might not prove committed to democracy.