(Adds dropped words in paras 10 and 11)
CAIRO, July 26 (Reuters) - Egyptian authorities have detained deposed President Mohamed Mursi for 15 days over an array of accusations, including killing soldiers and conspiring with the Palestinian group Hamas, the state news agency said on Friday.
The report came just hours before millions of Egyptians were expected to take to the streets in mass rallies for and against Egypt's first freely elected leader, who was ousted by the military on July 3.
Friday's step provided legal grounds for the continued detention of Mursi, who has been held by the military since he was ousted. The charges relate to his escape, along with other top Brotherhood leaders, from a prison north of Cairo.
The report on the state news agency said investigating judge Hassan Samir had confronted Mursi with evidence during his questioning. It did not say when or where he had been questioned.
A spokesman for the Muslim Brotherhood, which says the army has staged a coup against the democratically elected head of state, described the accusations as "ridiculous". Gehad El-Haddad said the move marked the return of the "old regime".
Mursi and many other Brotherhood leaders were rounded up by the authorities during the 2011 uprising that swept Hosni Mubarak from power.
The accusations listed against Mursi included arson, destruction of prison records and "collaboration with Hamas to undertake aggressive acts in the country, attacking police facilities, officers and soldiers".
It also accused him of "killing some prisoners, officers and soldiers deliberately and with prior intent". It added the accusation of "kidnapping some officers and soldiers".
The authorities have issued a gag order stipulating the media may only publish official statements on the case, citing the secrecy of the investigations and national security.
Hamas spokesman Sami Abu Zuhri said the probe marked a "grave development", adding that the new government "viewed the Palestinian cause as hostile".
"We challenge the current rulers of Egypt to bring a single piece of evidence on their alleged claim Hamas has intervened in internal Egyptian affairs," he told Reuters. (Reporting by Tom Perry in Cairo and Nidal al-Mughrabi in Gaza; Writing by Crispian Balmer; Editing by Michael Georgy)