By Opheera McDoom

KHARTOUM, July 8 (Reuters) - A human rights lawyer defending more than 30 people accused of participating in a Darfur rebel attack on Sudan's capital said on Tuesday he had been detained by state security and warned to drop the case.

"I was arrested by the Sudanese security for seven hours yesterday," lawyer Satie al-Hajj told Reuters. "They said this is a first and last warning for you: don't carry on with this cause and don't say anything to the media."

Hajj is one of a team of prominent Khartoum human rights lawyers who have petitioned the constitutional court to stop the trials. They say three special anti-terrorism courts formed to judge the accused are illegal and unconstitutional.

Sudanese security said they were checking the report.

Hajj said state security officials had demanded: "You are Arab. Why do you defend these people from the Zaghawa?" He said he would return to court later on Tuesday to try to continue his defence.

The non-Arab Zaghawa tribe is one of the three main tribes from which Darfur's rebels have drawn their ranks since taking up arms in early 2003, accusing central government of neglect. The tribe straddles Sudan's border with Chad, whose president, Idriss Deby, is also Zaghawa.

The attack in May was the first time a Sudan rebel force had carried its battle to the capital. About 200 people were killed and hundreds injured in clashes between the Darfur rebel Justice and Equality Movement (JEM) and government forces.

The accused face charges which carry the death penalty.

The government captured many rebels including JEM leader Khalil Ibrahim's brother-in-law and senior commander Abdel Aziz el-Nur Ashr who appeared in court on Thursday. Defence lawyers say they have not had access to him as yet.

Sudan has a "rehabilitation camp" for around 90 children it says were also involved in the attack, although JEM denies it has any child soldiers. The International Committee for the Red Cross (ICRC) says it has had free access to the children.

"The authorities approached the ICRC in Sudan almost immediately after they discovered they were minors and we have been able to do whatever we have asked to do," ICRC spokesman Saleh Dabbakeh said.

The ICRC had delivered messages from the children to their families, he said. Sudan says the children were mostly Chadian or Darfuris taken from refugee camps in eastern Chad, a charge JEM also denies.

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