By Maggie Fick and Yasmine Saleh

CAIRO, June 23 (Reuters) - A man was shot dead in overnight clashes north of Cairo between supporters and opponents of Egypt's Islamist government, security sources said, raising tensions ahead of big opposition rallies planned next Sunday.

A second man, also an Islamist supporter, died of gunshot wounds sustained in clashes south of the capital some days before and rival parties traded blame for the violence. The ruling Muslim Brotherhood described both dead men as "martyrs".

Highlighting mutual mistrust as Egypt struggles to establish democratic institutions after its 2011 revolution, the Brotherhood also denounced as a "political trial" a court judgment on Sunday that called for an investigation of its role in a mass jail-break during the uprising against Hosni Mubarak.

The once banned Brotherhood, whose Mohamed Mursi became president a year ago during a series of electoral triumphs that gave the well disciplined movement control of the executive and legislature, sees many in the judiciary as loyal to its foes.

It has criticised a campaigning coalition of disparate, secular opposition groups which hopes to force him to resign by putting millions on the streets on June 30 as an attack on the legitimate government, but says peaceful protests can go ahead.

Liberals and secular activists in the "Tamarud - Rebel!" campaign, accuse Islamists of intimidation. They say they have gathered 15 million signatures on a petition calling for Mursi to resign - more than the 13 million votes that elected him.

The Brotherhood and its Islamist allies staged a massive rally in support of Mursi in Cairo on Friday, at which some speakers warned of a violent response to efforts to remove him.

An opposition spokesman, Khaled Dawoud, said such rhetoric was fuelling the trouble in the streets.

TWO DEAD

The Muslim Brotherhood said on Facebook on Sunday that Karim Abdel Ghani, a member of the Islamist Nour party, was shot dead in Mahalla, an industrial city north of Cairo, by the "Tamarud militia" during clashes in his neighbourhood late on Saturday. Nour said its office in Mahalla was also attacked.

The Brotherhood's political wing, the Freedom and Justice Party (FJP), said it planned a funeral on Sunday for Mohamed Shalaqany, who it said was shot some days before by Tamarud "thugs" in Fayoum, a rural Islamist bastion south of Cairo.

"Their name is Tamarud but they are actually remnants of the old regime," Murad Ali, a senior FJP official, told Reuters.

Egyptian media gave extensive coverage on Sunday to a ruling by a judge in Ismailia who, in acquitting a man accused of fleeing a local jail during the 2011 uprising, asked the public prosecutor to investigate what he described as a "conspiracy" by the Brotherhood and foreign Islamists to open up the prison.

Among those freed was Mursi himself, who had been among hundreds of Brotherhood leaders rounded up as a precaution by Mubarak's security forces when the revolution began.

The freeing of Palestinian militants from Hamas and Lebanese members of the Shi'ite Hezbollah militia, among others, has prompted accusations from the Brotherhood's opponents that it connived with enemies of Egypt during the incident.

After the judgement, opponents of Mursi gathered outside the court, calling for his resignation. (Writing by Alastair Macdonald; Editing by Robin Pomeroy)