By Cynthia Johnston

CAIRO, Feb 13 (Reuters) - U.S. peace activist Cindy Sheehan pressed Egypt on Wednesday to free members of the opposition Muslim Brotherhood who are on trial in a military court, urging leniency for the sake of their families.

Egypt sent 40 members of the Brotherhood including its No. 3, Khairat el-Shater, to a military court a year ago. The men, six of whom are being tried in absentia, are the first Brotherhood members to face military trials since 2001.

Most of the serious charges, including terrorism and money laundering, were dropped in December, but the men are still charged with belonging to a banned group. A verdict is due on Feb. 26.

"I'm a mother whose son was killed in Iraq. I know what these families are going through -- stress and trauma," said Sheehan, who became a leading U.S. anti-war activist after her son Casey was killed in Iraq in 2004.

On a visit to Cairo to protest against the military trials, she added: "So as a mother, I'm asking for leniency from the Egyptian government, not only for the Muslim Brotherhood people, but for their children and their families also."

Dozens of police kept Sheehan and 200 Egyptian protesters cordoned off on a Nile river promenade in central Cairo as foreign tourists gawked, but they did not stop the protest.

Speaking over a loudhailer opposite government and ruling party offices, Sheehan accused Egypt of using the military court "as a last resort to guarantee a guilty verdict" and said U.S. aid to Egypt should be tied to better human rights practices.

Local and international rights groups including Amnesty International have criticised the military trial as unfair. Egypt has barred independent observers from the court.

"Freedom, where are you?" read a banner held by one protester. "We miss you, dad" read another, with a photograph.

Several protesters, mostly the wives and children of the Brotherhood detainees, held banners or wore shirts emblazoned with slogans comparing the children of the Muslim Brotherhood detainees to Sheehan's son. "She lost her son, we lost our husbands and fathers," said Brotherhood detainee Khairat el-Shater's daughter Zahraa, 30.

Aisha Hassan Malek, the 11-year-old daughter of one of the Brotherhood men, said: "I just want them to be found innocent and return to us and their families as soon as possible." The non-violent Muslim Brotherhood, which holds a fifth of the seats in Egypt's parliament, seeks an Islamic state through democratic means and operates openly despite a decades-old ban.

The government often detains Brotherhood members without charge, and a spokesman for thePublish Post group said nearly 350 are currently in detention. (Writing by Cynthia Johnston, editing by Tim Pearce)