(Updates with detention of activists, 15 Fatah men released)

GAZA, Sept 8 (Reuters) - Jaysh al-Ummah, a pro-al Qaeda group, called on Monday for Hamas to release its leader, Abu Hafss, who was detained last week after an armed training session in the Gaza Strip.

"He is a fighter and should not be jailed," a masked spokesman for Jaysh al-Ummah told a news conference in the Hamas-ruled territory.

The spokesman said the group would use "dialogue and not guns" to seek Abu Hafss's release.

Abu Hafss, 35, did not return home after being ordered by Hamas internal security to report for questioning last Wednesday, two days after some 25 Jaysh al-Ummah gunmen participated in a training session to which Reuters reporters were granted access.

The Hamas-run Interior Ministry in Gaza had no comment on Abu Hafss's arrest and acted quickly by sending security men to detain the three masked men who participated in the call for his release when they finished delivering their statement.

Abu Hafss vowed in an interview during the training event to fight Israel and "infidels". He also criticised Hamas for not implementing Islamic law in the Gaza Strip.

Jaysh al-Ummah says it supports al Qaeda ideology but is not part of the group itself.

Separately, Hamas, which took control of the Gaza Strip last year when it ousted the more secular Fatah faction of Palestinian President Mahmoud Abbas, released 15 detained Fatah men in a move it said was a goodwill gesture for Ramadan. (Writing by Nidal al-Mughrabi, Editing by Richard Williams)

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