GENEVA, July 19 (Reuters) - The top United Nations human rights official has asked Egypt's interim government to explain why the army detained ousted President Mohamed Mursi and his close aides and whether it plans to put them on trial, a U.N. spokesman said on Friday.

Mursi's Muslim Brotherhood allies have called for mass protests on Friday in their campaign to reverse the military's overthrow on July 3 of the elected head of state.

Mursi and his aides are being held at an unknown location.

Hundreds of his followers have been rounded up and arrest warrants have been issued for most of the Brotherhood's leaders. Some were charged with inciting violence after a July 8 clash that saw troops shoot more than 50 Mursi supporters dead.

The U.N. High Commissioner for Human Rights, Navi Pillay, met Egypt's ambassador in Geneva 10 days ago and pressed for information on Mursi and his team, especially the legal basis for their detention.

She has asked for a full list of people whose arrest was ordered and the names of those now in custody, Pillay's spokesman Rupert Colville said at a news briefing in Geneva. He said the Egyptians had given no reply.

"We've specifically asked about (Mursi) and his presidential team in addition to others who were arrested. We don't even know how many people at this point," said Colville.

Pillay told the new government she wanted to send a team to Egypt to learn more about its investigation into the shootings and the methods being used, he said.

"We're waiting for the approval of the authorities and a team is on standby ready to be deployed immediately as soon as such approval is received," Colville said.

The team also wants to advise the government on constitutional reforms. (Reporting by Stephanie Nebehay; editing by Tom Pfeiffer)