By Alastair Macdonald and Gwladys Fouche

OSLO (Reuters) - Declaring women's rights vital for world peace, the Nobel Committee awarded its annual Peace Prize on Friday to three indomitable female campaigners against war and oppression -- a Yemeni and two Liberians, including that country's president.

Ellen Johnson-Sirleaf, the first woman freely elected as a head of state in Africa, shared the award worth $1.5 million with compatriot Leymah Gbowee, who promoted a "sex strike" among efforts to end Liberia's civil war, and Yemen's Tawakul Karman, who called her honor "a victory for the Arab Spring."

"We cannot achieve democracy and lasting peace in the world unless women obtain the same opportunities as men to influence developments at all levels of society," Norwegian Nobel Committee chairman Thorbjoern Jagland told reporters.

"This is to highlight an incredibly important issue all over the world but especially in Africa and in the Arab world."

Karman, a 32-year-old journalist and veteran campaigner, has been a key figure in protests in the capital Sanaa this year: "This is a victory for the Arab Spring in Tunis, Egypt, Libya, Syria and Yemen," she told Reuters. "This is a message that the era of Arab dictatorships is over."

Typically, she was out demonstrating in a central square in Sanaa for the departure of veteran Yemeni President Ali Abdullah Saleh when she heard the news.

Johnson-Sirleaf, 72, a former World Bank economist dubbed the "Iron Lady" by opponents, called it a recognition of her nation's "many years of struggle for justice, peace, and promotion of development" since a brutal decade of civil war.

"I believe we both accept this on behalf of the Liberian people, and the credit goes to the Liberian people," she said.

Gbowee, 39, was traveling in the United States. Her Women For Peace movement is credited by some helping end the war in 2003. Starting with prayers and songs at a fish market, she also urged the wives and girlfriends of leaders of the warring factions to deny them sex until they laid down their arms.

ARAB SPRING WARNING

Jagland rejected suggestions the panel might skew Liberia's election on Tuesday by giving Johnson-Sirleaf a boost from the prize in her bid for a second term.

But he called the award to Karman a signal to Arab autocrats that it was time to go, as well as a warning to new leaders, including Islamists, to protect women's rights.

"If you look at the Arab Spring, this is a crucial issue," Jagland told Reuters. "Unless they include the women in the development there, then they will fail ... I am worried about what is going on in several of these countries.

"So this is a clear message to those who are trying to build democracy -- that you have to take the women on board."

Islamists have emerged strongly from the shadow of secular autocrats in Tunisia, Egypt and Libya and are well represented in opposition movements in Yemen, Syria and elsewhere. Jagland, a former Norwegian prime minister, said he was not criticizing Islam but warning against abusing religion to oppress women.

The trio named by the Norwegian committee, whose other four members are all women, follow only a dozen women among 85 men to have previously won the prize over its 110-year history.

The panel hoped the award would help end "the suppression of women that still occurs in many countries, and to realize the great potential for democracy and peace that women can represent."

The committee said all three women were rewarded from the bequest left by Swedish dynamite inventor Alfred Nobel for "their non-violent struggle for the safety of women and for women's rights to full participation in peace-building work."

WORLD WELCOME

The prize was less controversial than in the last two years. In 2010, China was outraged and imposed sanctions on Norway after the award went to jailed dissident Liu Xiaobo.

The award to Barack Obama in 2009, just months after he became U.S. president, amazed many, not least the new occupant of the White House. It recognized, among other things, promises he made of promoting democracy in the Arab world.

Many Arabs were disappointed by what they saw as Obama's slowness in switching Washington's allegiance from rulers like Egypt's Hosni Mubarak and Yemen's Saleh, a U.S. ally against al Qaeda, to the crowds of demonstrators trying to oust them.

Obama's predecessor, as both a Democratic president and a Peace Prize laureate, Jimmy Carter urged him to make good on promises, including about democracy in the Middle East.

"It was given primarily because of some of the commitments he had made verbally," Carter told Reuters on Thursday. "I hope that some of those promises will be realized."

Among those who welcomed this year's award was German Chancellor Angela Merkel, viewed as the most powerful woman in Europe, who said: "This will hopefully encourage many women, but also many men, around the world to campaign for freedom and democracy and against injustice."

U.N. Secretary-General Ban Ki-moon said: "Above all, it underscores the vital role that women play in the advancement of peace and security, development, and human rights."

Amnesty International secretary general Salil Shetty said: "This Nobel Peace Prize recognizes what human rights activists have known for decades - that the promotion of equality is essential to building just and peaceful societies worldwide."

Egyptian activist Asmaa Mahfouz, among a number of other Arab women protesters who had been widely tipped, said: "Giving it to Yemen means giving it to the Arab Spring, and this is an honor to all of us and to all Arab states."

CAMPAIGN BOOSTS

Karman vowed to end the rule of Saleh, which began a year before she was born. "Our peaceful revolution will continue until we topple Saleh and establish a civilian state," she said.

The Peace Prize was widely welcomed as shot in the arm for their movement by fellow protesters, even though many had begun to tire of what some complained was a "dictatorial" style of her own: "She's a controversial figure for the protesters, but either way everyone is happy today," said youth activist Atiaf al-Wazir. "This is a sign the world supports our peaceful protest movement, people feel the world is standing with us."

Johnson-Sirleaf also has no shortage of critics, and the prize may have bolstered her chances of securing immediate re-election in Tuesday's first round of the presidential vote, where she has been involved in a tight three-way race.

Opponents have said she had not done enough to heal the West African nation's divisions. She has admitted providing food, supplies and money to rebel leader and indicted war criminal Charles Taylor in the early years of the civil war, something her critics say undermines her image as a bringer of peace.

Gbowee's brother, Alphonso Diamond Gbowee, told Reuters: "I am so excited that her relentlessness to ensure the development of women and children in our region has been recognized.

"She's very hard-working, helping with women and children all over the place, especially in Ghana, Liberia and Sierra Leone ... This will be a challenge for her to do more. I have no doubt she'll continue to impact those vulnerable lives."

The three will presented with the prize in Oslo on December 10, the 115th anniversary of the death of benefactor Alfred Nobel.

(Additional reporting by Victoria Klesty, Walter Gibbs, John Acher, Joachim Dagenborg, Camilla Knudsen and Terje Solsvik in Oslo, Richard Valdmanis and Mark John in Dakar, Alphonso Toweh and Clair MacDougall in Monrovia, Kwasi Kpodo in Accra, Yasmine Saleh in Cairo, Andrew Hammond in Dubai, Erika Solomon, Mohammed Ghobari and Ahmed Jadallah in Sanaa and Samia Nakhoul in London; Writing by Alastair Macdonald)