Angelina Jolie said a global summit to end sexual violence must send a message around the world that there is "no disgrace" in being a survivor - "that the shame is on the aggressor".
On her arrival at the summit in east London, the Hollywood star dedicated the four-day event to an unnamed and "abandoned" victim of rape in Bosnia.
Foreign Secretary William Hague also announced that the UK will pledge a further £6 million to support survivors of sexual violence in conflict.
Mr Hague said it is only a "weak or inadequate man" that abuses women - a statement that led to cheers from the crowd.
In a speech that was met with loud applause, Jolie, who is special envoy for the UN High Commissioner for Refugees, said: "We must send a message around the world that there is no disgrace in being a survivor of sexual violence, that the shame is on the aggressor."
She said it is a "myth" that rape is an inevitable part of conflict.
"There is nothing inevitable about it - it is a weapon of war aimed at civilians.
"It has nothing to do with sex, everything to do with power," the Tomb Raider star said.
The actress told the packed crowd at the opening of the End Sexual Violence in Conflict (ESVC) summit that she has met survivors in countries including Afghanistan and Somalia, and they are "just like us, with one crucial difference".
She said: "We live in safe countries with doctors we can go to when we're hurt, police we can turn to when we're wronged, and institutions that protect us.
"They live in refugee camps, on bombed-out streets, in areas where there is no law, no protection, and not even the hope of justice."
Jolie said the international community needs to work to make "justice the norm".
She called for all armies, peacekeeping troops and police forces to have prevention of sexual violence in conflict as part of their training.
"This whole subject has been taboo for far too long," she said.
Jolie said the stigma causes survivors to suffer feelings of "shame" and "worthlessness".
"It feeds ignorance, such as the notion that rape has anything to do with normal sexual impulses.
"But most of all it allows the rapist to get away with it. They feel above the law because the law rarely touches them and society tolerates them," she said.
Speaking as she arrived at the ExCeL conference centre in London's Docklands, the actress spoke about how she and Mr Hague met a woman earlier this year during a campaigning trip to Bosnia.
She said the woman was too humiliated to tell her own child that she had been raped - and had seen her attacker "on the streets, free".
Speaking to the press, Jolie said: "I am so, so happy to be here, it has been long in coming, we have worked on this for quite a while.
"On our way over, we spoke about the women we met recently on our last trip, and in particular one woman, who said that she had yet to tell her child that she had been raped because she was so humiliated and she could not bring herself to admit it to him.
"And she felt that, having had no justice for her particular crime, in her particular situation, and having seen the actual man who raped her on the streets, free, she really felt abandoned by the world.
"On the way over, we thought 'What is she going to think of this day?'. This day is for her."
The 39-year-old star, dressed in white, was followed around the summit by l arge numbers of people holding their camera-phones aloft, keen to get a photograph.
The ESVC conference - with 117 countries represented - aims to draw up an international agreement on standards for documenting and investigating sexual violence in conflict zones in an attempt to ensure justice for victims.
Campaigners are also pressing for armies to be trained to prevent sexual violence in conflict zones and for more support for survivors.
The Foreign Secretary said he and Jolie began campaigning two years ago because they "believe the time has come to end the use of rape in war once and for all".
The pair will co-chair the four-day summit.
In his opening speech, Mr Hague said: "We're convinced this is an issue of international peace and security, that is central to conflict prevention, that it is fundamental to the advancement of women's rights everywhere, and above all that it is a moral issue for our generation."
He said the facts are now beginning to emerge for all to see.
"What would it say about Britain, or any other nation, if, knowing all this, we chose not to act, we chose to do nothing?
"As was said with slavery in the 18th century, now we know the facts, we cannot turn aside."
Mr Hague said he wants the summit to "shatter the impunity of sexual violence".
He said there needs to be a "change in attitudes" - to shift the stigma from the survivors to the perpetrators.
To much applause, he said: "We want to encourage men to speak out, to agree with us that it is only a weak or inadequate man who abuses women.
"It's not a sign of strength. It is the ultimate weakness and shame."
Speaking on day one of the summit, Prime Minister David Cameron said: "This Government will always seek to protect the most vulnerable in society, both at home and abroad.
"Both in the Modern Slavery Bill and in our campaign against sexual violence in conflict we are protecting victims and punishing perpetrators - with tougher sentences for traffickers and ending impunity for soldiers who commit rape.
"Britain should not and will not tolerate trafficking, and sexual violence has no place as a weapon of war in this world."
£5 million of the £6 million pledged by Mr Hague will come from the Department for International Development (DfID). The extra £1 million will come from the Foreign Office, DfID said.