"We are here for the long term and what we can offer is support to Libyan institutions and the economy. We will be here to support you all the way," Ms Ashton said in the rebel stronghold of Benghazi, where she opened an EU representative office.
France, Britain and other European states have backed Libya's poorly trained and equipped rebels against a government that has held on to power for more than four decades.
French planes were the first to bomb Col Gaddafi's forces in March after the United Nations voted to allow intervention to protect civilians. The air strikes, now led by Nato, were launched as Col Gaddafi's troops advanced on Benghazi after the Libyan leader vowed "no mercy, no pity".
"I'm very clear that protecting civilians and the people of Libya is fundamental," said Ms Ashton. "Too many people have died already, it is important to realise that Gaddafi should leave."
Ms Ashton's visit "shows the increased support of the European Union in supporting us to have a democratic and free state", said the head of the Benghazi-based National Transitional Council, Mustafa Abdel Jalil.
"The Libyan people appreciate this visit and appreciate the European Union for supporting the revolution," he said.
Nato airstrikes that began on 31 March have prevented Col Gaddafi's tanks from overrunning rebel-held towns. But an uneasy stalemate has since settled on the battle space with a nearly static frontline in the east and fighting in western Libya.