* Legal action targets family viewed as Brotherhood supporters
* Brotherhood banned, thousands of backers in jail
* Group loses grip on powerful professional syndicate (Adds Brotherhood's losses in Doctors Syndicate elections)
By Maggie Fick
CAIRO, Dec 15 (Reuters) - A prosecutor has ordered the arrest of an Egyptian man whose 15-year-old son was detained last month for owning a ruler bearing a symbol associated with the Muslim Brotherhood, the family's lawyer said on Sunday.
Mohamed Abdulghani Bakara was accused of "inducing" his son Khaled to take the ruler to school, the lawyer said, the latest sign of a widening crackdown on the organisation since the army ousted President Mohamed Mursi of the Brotherhood in July.
Two of the boy's teachers, Ashraf Raslan and Hamidou el Kheish, also faced charges of "spreading chaos among school students" by inducing him to possess the ruler, judicial sources said.
Hundreds of Brotherhood members have been killed by security forces since Mursi's overthrow. Authorities have jailed thousands more.
Heba Morayef of campaign group Human Rights Watch said the case illustrated the "complete arbitrariness" of arrests since Mursi's overthrow and risked intimidating voters ahead of a referendum on whether to accept a new constitution, scheduled for Jan. 14 and 15.
"It shows the targeting of people not even on the basis of membership (in the Brotherhood), but also based on perceived allegiance or affiliation," said Morayef.
Lawyer Amr Abdel Maksoud told Reuters there was no legal basis for the charges and arrest warrants issued in the Nile Delta town of Kafr el-Sheikh.
"They (the prosecution) are helping the army dominate the country," he said.
Khaled remains in detention on suspicion of inciting violence, slandering the Egyptian army and membership of a banned group, legal sources said.
The vote on the constitution, which will replace the one passed during Mursi's presidency, paves the way for new parliamentary and presidential elections.
The Brotherhood, which won every election since the 2011 uprising that toppled Hosni Mubarak, dismisses the army-backed government's plan for a return to elected rule as an attempt to paper over a coup.
In another sign of strain on the movement, the Brotherhood lost its grip on a powerful professional union it had controlled since the 1980s.
In elections for vacant posts in the Doctors Syndicates held on Friday, a coalition of non-Brotherhood candidates won 11 of the 12 seats on the syndicate's board, official results showed.
Syndicates have traditionally been seen as a gauge of Brotherhood support, in large part because the group was banned from politics during the Mubarak era.
The syndicates provided a way for Brotherhood members to participate in public life when "all the doors and windows were closed", said Mohamed Osman, one such member who lost his bid for re-election.
Osman said he and other members of the Brotherhood's Doctors for Egypt group in the syndicate accepted the results, but put the defeat down to the security crackdown against the movement.
"Most of our supporters and most of our previous syndicate leaders are under arrest or are in hiding for their safety," he said. (Editing by Will Waterman)