* Activists say latest sign of government crackdown
* Dozens of cases of "insulting the president" brought
* No comment from dominant Muslim Brotherhood group
By Maggie Fick
CAIRO, April 30 (Reuters) - A prominent Egyptian campaigner was detained on Tuesday over accusations of insulting President Mohamed Mursi, state media reported, a case activists said highlighted a crackdown on dissent by the Islamist government.
A court in the town of Tanta ruled Ahmed Douma should be held for four days, pending investigation into charges of "deliberately spreading false news and rumours" during two recent television appearances, a court official told Reuters.
Douma was accused of calling Mursi a criminal and a murderer, alleged statements the charge sheet said could damage the public opinion of the president.
Dozens of cases of "insulting the president" have been brought in Egypt since Mursi took office last June. Activists said the case was the latest example of the government using the courts to target its liberal and secular opponents.
"They feel that those activists who criticize the president are, according to their words, harming the president, so they try to silence them," said Hafez Abu Seada, head of the Egyptian Organisation for Human Rights.
Ziad el-Elaimy, a liberal politician who served in the parliament dissolved last year wrote a statement of support for Douma on his Twitter feed and said Egypt's dominant Muslim Brotherhood group - which propelled Mursi to power - was behind the case.
"The Muslim Brotherhood sees a threat from the revolutionary youth and the opposition and wants to control them," Douma's laywer Ali Soliman told Reuters, who said that man who filed the charge against Douma was a member of the Brotherhood's political arm, the Freedom and Justice Party.
A Brotherhood spokesmen did not respond to phone calls on Tuesday from Reuters requesting comment.
Television satirist Bassem Youssef, who is often compared to American "Daily Show" comedian Jon Stewart, was summoned by the state prosecutor for questioning last month for allegedly insulting Mursi and Islam.
Shortly after Youssef appeared in court, the United States said it was concerned about "disturbing trend of growing restrictions on freedom of expression".
Douma was one of five activists served with arrest orders last month by the prosecutor general's office on accusations that included inciting aggression during street clashes near the Muslim Brotherhood headquarters.
Douma and three other activists refused to show up for questioning and their case was later dismissed, while prominent blogger Alaa Abd El-Fattah was released after being questioned.
(Additional reporting by Shaimaa Fayed; Writing By Maggie Fick; Editing by Andrew Heavens)