David Cameron and Barack Obama have urged the Libyan leader to stand down, saying they would "turn up the heat on the Gaddafi regime".
The two men have held a joint news conference in London after discussions at Downing Street and ahead of a speech by the US President to both Houses of Parliament.
Mr Obama said: "We will continue to strongly oppose the use of violence against protesters and any efforts to silence those who yearn for freedom and dignity and basic human rights - that's one of the reasons we are working together in Libya alongside our Nato allies and partners to protect the Libyan people.
"We will continue those operations until Gaddafi's attacks on civilians cease. Time is working against Gaddafi and he must step down from power and leave Libya to the Libyan people."
Mr Cameron joined the president in urging Col Gaddafi to quit, saying: "It is impossible to imagine a future for Libya with Gaddafi still in power. He must go...we want to turn up the heat on the Gaddafi Regime."
The Prime Minister added that they had held "excellent discussions" and described the relationship betweent the two countries as "essential".
The Prime Minister said: "I feel everyday just how important this partnership is.
"It is a living, working partnership and it is essential to our security and it is essential to our prosperity."
Mr Obama told journalists: "The relationship between our cournties is built on shared ideals and shared values...it is stronger then it has ever been."
Later the President will insist the alliance between the US and UK is key to providing global security in a speech to MPs and peers.
According to the White House, Mr Obama is expected to say that even though the world has changed significantly since World War Two, the UK-US relationship and the broader transatlantic alliance is still the "cornerstone of global security".
The "Arab Spring" in the Middle East and North Africa reinforce that the allies' shared beliefs are "not just relevant but essential", he will say.
The President will also strike an optimistic note by claiming that the world is "turning a corner" following a "difficult decade".
As well as operations winding down in Iraq, the shift to Afghanistan taking control of its own security has begun, and al Qaeda has been "weakened" by successes such as the recent killing of Osama bin Laden.
Mr Obama follows Bill Clinton and Ronald Reagan to become the third US President to address Parliament.
The second day of the Obama's state visit to Britain began at Downing Street where he held talks with David Cameron on issues ranging from Libya and Afghanistan, to terrorism and the global economy.
Mr Obama was smiling broadly after making the short journey in Cadillac One - his bombproof limousine known as "The Beast" - from Buckingham Palace.
Mr Cameron and Mr Obama sat down for talks in the White Room of No 10 shortly before 10am.
As about a dozen photographers took pictures at the start of the meeting, the President joked: "All right guys, one of those must have worked."
While the President is in Westminster, Mrs Obama has travelled to Oxford to make a speech on the importance of education.
The US first lady is visiting the elite institution this afternoon and will catch up with pupils from the all-girl Elizabeth Garrett Anderson School (EGA).
Mrs Obama first met pupils from the north London secondary school when she accompanied US president Barack Obama on their first visit to London in April 2009.
She made a surprise trip to the school, where she urged the young women to "be the best that you can be" and to have "confidence and fortitude" to determine their own success.
On Tuesday Mr Obama and his wife Michelle were welcomed to the UK by the Queen who hosted a banquet for them at Buckingham Palace.
In a speech at the dinner Her Majesty celebrated the special relationship between the two countries and highlighed their "shared history, common language and strong intellectual and cultural links".
In response Mr Obama spoke of the "rock solid foundation" between the countries and thanked the UK for its solidarity since 9/11.
Read more stories about President Obama's visit:
:: Queen Welcomes Obamas At State Banquet
:: President Obama's Abbey Guest Book Gaffe
:: Huge Security Operation For Obama's UK Visit
:: Picture Gallery: Obamas Meet Royal Newlyweds
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