The International Criminal Court's prosecutor asked judges Thursday for permission to investigate Kenya's postelection violence that left more than 1,000 people dead.

Luis Moreno Ocampo said there is a "reasonable basis to believe that crimes against humanity" were committed when violence erupted after President Mwai Kibaki was declared winner of Kenya's 2007 elections.

The opposition claimed the vote was rigged, and protesters swarmed the streets of the Kenyan capital, Nairobi, between December 2007 and February 2008. Many protesters who clashed with police were killed, but the violence also erupted along tribal lines.

Moreno Ocampo said Kenyan authorities believe 1,220 people were killed, hundreds were raped and thousands forced from their homes.

"These alleged crimes were part of a widespread and systematic attack against the Kenyan civilian population," Moreno Ocampo said. "This constitutes crimes against humanity — murder, rape and sexual attacks, forced displacement and inhuman acts."

He said his investigators were ready to visit Kenya early next year to speak with victims.

If judges allow Moreno Ocampo to open an investigation, it would be the fifth since the court started work in 2002 — all of them based in Africa.

Thursday's announcement marks the first time the prosecutor has called on judges to open an investigation, and activists welcomed the step. In other cases, the countries involved or the U.N. Security Council have asked for the court to investigate.

"The ICC is a court of last resort, and when national authorities are unwilling to act, it is supposed to step in," said Elizabeth Evenson, counsel in the international justice program of Human Rights Watch. "Today's announcement shows that the ICC prosecutor can and will act on his own in situations of serious crimes."

Moreno Ocampo said he also was concerned about reports of intimidation and threats to victims and potential witnesses by Kenyan police. He said it would be difficult to protect witnesses.

Moreno Ocampo's office has been evaluating whether the Kenyan violence constituted crimes against humanity since January 2008. In July, former U.N. chief Kofi Annan, who mediated an end to the violence, sent Moreno Ocampo a sealed envelope with the names of suspected ringleaders named by an independent commission.

The commission kept those names secret, saying they were powerful individuals who could interfere with future investigations, but did say that a handful of Cabinet ministers, business people and police officers were listed.

Moreno Ocampo said he was not bound by the commission's findings and would conduct his own investigation. He said he hoped the inquiry would prevent violence during Kenyan elections in two years.

"This is an opportunity to rebuild Kenya on new foundations, solid foundations," he said.