GENEVA (Reuters) - Egyptian President Mohamed Mursi's decree granting himself extended powers raises very serious human rights concerns, U.N. Human Rights Commissioner Navi Pillay's spokesman said on Friday.

Mursi's announcement was welcomed by his allies but prompted fears among secular-minded Egyptians that the ruling Muslim Brotherhood aimed to dominate the new Egypt.

Among its other provisions, the decree states that all decisions taken by Mursi until the election of a new parliament are exempt from legal challenge. It also orders a retrial of former President Hosni Mubarak and his aides.

"We are very concerned about the possible huge ramifications of this declaration on human rights and the rule of law in Egypt," Pillay's spokesman Rupert Colville told a news briefing at the United Nations in Geneva.

"We also fear this could lead to a very volatile situation over the next few days, starting today in fact."

Colville did not specify which parts were most worrying but said the decree had many aspects to it so it would take time to analyze fully.

He said Pillay's office may put out a full statement later on Friday or on Saturday.

(Reporting by Tom Miles; Editing by Angus MacSwan)