A Congolese gynaecologist known for his work with victims of sexual violence has told how a quick-thinking friend saved him from assassination and paid with his own life.

Denis Mukwege, who has won numerous awards and been nominated for the Nobel peace prize, narrowly escaped death when five gunmen in civilian clothes attacked his home in Bukavu, eastern Democratic Republic of the Congo, on Thursday night.

"I thought they would kill me," he told the Guardian. "These people got into my home and my two daughters were there. They told them to sit down and not use the phone. They sat with them in the dining room and waited until they heard my car drive up."

One of the assailants opened the gate and forced Mukwege out of his vehicle. "I was at one side of the car and the man was at the other side," Mukwege continued. "He came to me and tried to shoot me but Jeff [a man employed at Mukwege's home] was behind him and called out. The man turned around and shot him two times."

Out of instinct Mukwege threw himself to the ground. "He turned back to shoot me but I was already down. He jumped in the car and left quickly."

The gunmen are believed to have swiftly abandoned Mukwege's car before hijacking another vehicle. Their identity and location are unknown.

Mukwege, 57, is relieved that his family are unscathed but mourning the death of the man he always knew simply as Jeff, a trusted friend and employee who was the same age as him. "The man worked with me a long time," he said, his voice cracking with emotion. "It's a terrible loss for me and all my family."

The motive for the assassination attempt is a mystery. "Why? I don't know. They didn't ask me for money or anything."

Mukwege is the founder and medical director of the Panzi hospital in Bukavu, where he and his staff have helped treat more than 30,000 survivors of sexual violence – often gang rapes – by armed groups, including women in need of specialised gynaecological care. He has been described as "likely the world's leading expert on repairing injuries of rape."

Mukwege also travels regularly to speak about the plight of these victims and he addressed the UN general assembly in September.

His work has earned numerous international awards including the UN Human Rights Prize, King Baudouin Africa Development Prize, African of the Year, Olof Palme Prize and Clinton Global Citizen Award, while the Carter Foundation has named him a "citizen of the world".

Word of the murder attempt caused anger and revulsion around the world. Activist and playwright Eve Ensler, founder of the V-Day global movement to end violence against women, who first travelled to Congo at Mukwege's invitation in 2007, said: "One of the great men of the world was almost murdered tonight. We cannot let this continue, we must create an environment where it is safe for Dr Mukwege and all the people of Congo to live and thrive."

There was speculation of a political motive for the attack. Describing Mukwege as "one of my heroes", New York Times columnist Nicholas Kristof blogged: "Dr Mukwege presumably was targeted because of a strong speech he gave at the United Nations last month, denouncing mass rape in Congo and the impunity for it. President [Joseph] Kabila has long been angry at Dr Mukwege, and the UN speech can't have helped. Meanwhile, Dr Mukwege has also offended Rwanda with his denunciations of Rwanda's role in the slaughter and rape in eastern Congo."

The Sonke Gender Justice Network, a non-government organisation working for equality in Africa, said: "We fear for the safety of Dr Mukwege and worry that this attempt on his life was directly linked to his advocacy work. It closely follows a speech he gave at the UN last month in which he denounced the country's 16 years of violent conflict and called for 'urgent action to arrest those responsible for these crimes against humanity and to bring them to justice.'

"We call on the DRC [Congolese] government to immediately ensure his safety. We also call on each of our own governments and the African Union to monitor the situation and to ensure Dr Mukwege's safety."