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Khieu Samphan, left, and Nuon Chea, right, attend the funeral for the former wife of Pol Pot in Pailin, 2003. (AP Photo)

Khieu Samphan, the former nominal head of the Khmer Rouge, was charged with war crimes and crimes against humanity late Monday, following his arrest in a Phnom Penh hospital earlier that day.

Police escorted the aging leader from Calmette Hospital, where he was recovering from a stroke in Pailin, and after a day of questioning, the Khmer Rouge tribunal formally charged him, for his role under the regime.

"Khieu Samphan's arrest today means that the court has fulfilled the first step," tribunal spokesman Reach Sambath said. "His arrest today means that the co-judges have finished the first step in the investigation."

"This is a sign that Cambodia is going forward under the support of the UN, to stop impunity," he added.

Khieu Samphan, 76, is the fifth on a five-person list to face trial in the first round of a UN-backed, hybrid tribunal. Khieu Samphan's Cambodian lawyer, Say Bory, declined to comment after the arrest Monday.

The arrest was roundly congratulated, though some observers warned the tribunal, which has experienced infighting, accusations of mismanagement and an oft-delayed beginning, was not out of the woods.

"I see there will be problems in the future," said Hisham Mousar, a legal expert and tribunal monitor for the rights group Adhoc.

Following this first round, the tribunal could hit "deadlocks" as it attempts more indictments, he said. "The national and international co-prosecutors might not reach a consensus on [other] top former Khmer Rouge leaders becoming suspects," he said.

That would mean investigating judges would be working on just five cases of leaders: Khieu Samphan, 76, former foreign minister Ieng Sary, 78, social affairs minister Ieng Thirith, 75, chief ideologue Nuon Chea, 82, and torture center chief Kaing Khek Iev, 64, alias Duch.

US Embassy spokesman Jeff Daigle called the arrest "another positive step in the court proceedings."

"The arrest shows that the Khmer Rouge tribunal is increasing its speed in pushing the procedures to go faster, in taking top Khmer Rouge leaders to be responsible before history and their victims," said Thun Saray, director for the rights group Adhoc.

Human Rights Party Vice President Keo Remy appealed to the tribunal to "find justice" for leaders who "took no responsibility during their genocidal regime."

"Comrade Khieu Samphan says he knows nothing at all," Keo Remy said. "I say this is irresponsibility." Khieu Samphan's arrest "is ending the conflict…military, political and diplomatic," said Youk Chhang, director of the Documentation Center of Cambodia, which has worked for a decade to document the atrocities of the Khmer Rouge.

"We hope that in the future, we do not want to have such leaders," said Tuol Sleng prison survivor Van Nath, who also welcomed Khieu Samphan's arrest. The upcoming trials of all the leaders could warn people about the past, about "the ones who mistreated the people, not only me, but all the people of Cambodia."

"I hope in the future, justice will be done," he said.