Mia Farrow will begin a hunger strike next week to show solidarity with the people of the war-hit Darfur region of Sudan, she has said.
The actress, 64, said she was fasting because the Sudanese government had decided to expel foreign aid agencies.
Thirteen aid groups were expelled in March after the International Criminal Court issued an arrest warrant for Sudan's president, Omar al-Bashir.
Mr Bashir is accused of orchestrating atrocities against civilians in Darfur.
The government has been fighting rebels in the region since 2003.
I undertake this fast in the heartfelt hope that world leaders who know what is just and right will call upon the government of Sudan to urgently readmit all of the expelled agencies
The UN says that up to 300,000 people have died during the conflict and 2.7 million driven from their homes.
Farrow said she wanted to express "outrage at a world that is somehow able to stand by and watch innocent men, women and children needlessly die of starvation, thirst and disease".
"I undertake this fast in the heartfelt hope that world leaders who know what is just and right will call upon the government of Sudan to urgently readmit all of the expelled agencies or otherwise insure that the [aid distribution] gap is filled," she said.
Farrow, who was once married to Frank Sinatra and later had children with Woody Allen, called on "human rights advocates and citizens of conscience around the world" to join her in "some form of fasting, even if for one day".
The actress, who is a goodwill ambassador for UN children's foundation Unicef, plans to start her "fast of only water" on Monday.
A spokesman for Farrow said she would fast for as long as possible.
Doctors estimated that, given her slim build, this would be for a maximum of three weeks, the spokesman added.
But Sudan's ambassador to the UN said that, instead of fasting, Farrow should use her fame "to put pressure on the rebel groups to come to the negotiating table".
"The government is committed to the welfare of its own citizens and doesn't need anybody to remind it of that," Abdalmahmoud Abdalhaleem told Reuters news agency.
"Darfur should cease to be an arena for those seeking fame and publicity."
One white woman not eating. This will change everything.