LONDON — Family and friends gathered at Westminster Cathedral Saturday to remember Margaret Hassan, a respected aid worker abducted and believed murdered in Iraq.

Cardinal Cormac Murphy-O'Connor, leader of Britain's 5 million Roman Catholics, celebrated the requiem Mass for a "gentle, private, brave, loving and compassionate" woman he called a martyr.

"I use the word advisedly, because the word martyr means witness," he said. "Margaret witnessed, in both her life and her death, to the act of loving."

A portrait of Hassan, who was kidnapped in October and whose body has not been found, was placed by the altar instead of a coffin. More than 2,000 mourners joined in hymns and prayers for Irish-born Hassan, who also held British nationality and had lived in Iraq for 30 years.

In a statement, Hassan's sisters and brother said they had been "overwhelmed" by the messages of sympathy received from around the world since a video purporting to show her death surfaced last month.

Hassan's Iraqi husband, Tahseen Ali Hassan, had been too ill to travel to Britain for the service, the British Broadcasting Corp. reported.

Family friend Patrick O'Ryan-Roeder, who delivered a tribute to Hassan at the service, remembered her as "a much-loved wife, a much-loved sister, a much-loved aunt."

"I know that our thoughts and prayers have always been with Margaret, and I believe they should also be extended to her immediate family as they try to come to terms with their devastating loss," he said.

The Iraq director of aid agency CARE International, Hassan, 59, was abducted in Baghdad on Oct. 19 on her way to work. Her captors later issued videos showing her pleading for Prime Minister Tony Blair to withdraw troops from Iraq and calling for the release of female Iraqi prisoners.

On Nov. 16, the Arab satellite television station Al-Jazeera said it had received a video showing a hooded militant shooting a blindfolded woman in the head. British officials said they believed the woman in the video was Hassan, and her family said they believed she was dead.

Murphy-O'Connor described Hassan as "a peacemaker in a time of seemingly endless wars. She hungered and thirsted for justice for the Iraqi people."

"She was persecuted, brutally slain, because she was working in the cause of right. Margaret is not merely a memory, she is part of all of us. She will not be forgotten."