CAIRO, June 3 (Reuters) - A high-profile Egyptian blogger and activist was sentenced to six months in jail on Monday for insulting President Mohamed Mursi, in what campaigners said was the first major conviction in a legal crackdown on critics.

More than 100 of Ahmed Douma's supporters filled the courtroom in a Cairo suburb and chanted slogans against the Islamist president during the hearing.

"It's clear that the government is trying to threaten activists with these cases," one of his lawyers, Ali Soliman, told Reuters.

Douma, found guilty of calling the president a criminal and a murderer in media interviews, was allowed to pay 5,000 Egyptian pounds ($720) bail to stay out of prison pending an appeal, Soliman added.

Mursi, voted in after a popular revolt ousted his predecessor Hosni Mubarak in 2011, has dismissed accusations by rights groups that his government and allies want to crush dissent.

But one Egyptian campaign group has said in a report two dozen cases of "insulting the president" were brought in the first 200 days of Mursi's rule - four times as many as during Mubarak's 30 years in power.

"The irony is that the president elected after the 25 January revolution is still maintaining the same restrictive laws that have been in place for decades," said Gamal Soltan, political science professor at the American University in Cairo.

Douma was arrested on April 30 on charges of insulting the president in the aftermath of deadly clashes in February between locals and police in the Suez Canal city of Port Said.

Mursi has pointed to his banning of pre-trial detention of journalists as proof of his commitment to a free press, though his government has not amended laws with a wide scope for prosecution on grounds of defamation.

"The main problem is the penal code," said Heba Morayef, Egypt director of the New York-based Human Rights Watch, after the case.

"It allows citizens to be locked up for expression-related crimes," punishing citizens for "legitimate political criticism of the authorities," she added.

($1 = 6.9853 Egyptian pounds) (Writing by Maggie Fick; Editing by Andrew Heavens)