As fierce fighting continues between rebels and Gaddafi loyalists in Tripoli, the sister of a passenger killed in the Lockerbie bombing is calling upon the dictator to relinquish power.

Lisa Gibson’s brother, Kenneth Gibson, was one of 270 people killed when Pan Am flight 103 was blown up in a terrorist attack over Lockerbie, Scotland, in December 1988.

Gibson, a Christian attorney who has met Colonel Muammar Gaddafi, made a heartfelt plea to the dictator to step down in an open letter sent to ASSIST news service.

“Please step down, because your reign of terror is up,” she said.

“Those that had supported you in Libya are now turning their back on you. They supported you out of fear, not love, respect or loyalty.

"Like an abused child who stands up for their abuser, they did so because that is what they have been required to do for the last 42 years of oppression.

“They are now realising that their Brother Leader is really not a good Brother to them after all.”

Gibson, who is a frequent visitor to Libya, said she could see “freedom, hope and joy” in the people for the first time.

“For years, the Libyan people lived in constant fear of your regime. But they are no longer afraid and they have found their voice.”

She continued: "So, in conclusion, I ask if there is any sense of humanity left in you that you would do the right thing. Leave before any more of my friends in Libya have to lose their lives.

"They have tasted freedom and are fueled by hope and conviction that you will never understand.

"Give them the opportunity to have the freedom, peace and prosperity that they desire."

Loyalists are clinging on to Tripoli after rebels swept in throughout Monday.

There are reports of many casualties as they continue to put up a fight in parts of the city.

There had been reports that Gaddafi’s son, Saif-al-Islam, was being held by rebels but he made a defiant appearance alongside loyalists in Tripoli in the early hours of Tuesday morning.

The whereabouts of Gaddafi are still not known.