Saddam Hussein has appeared before a judge in Baghdad after being transferred to Iraqi legal custody.
Saddam Hussein could face the death penalty, if convicted
The former president was "visibly nervous" when he was read his rights, said Salem Chalabi, head of the special tribunal in charge of the trial.
Saddam Hussein will appear in court with 11 other senior members of his former regime on Thursday, where they will hear the charges against them.
Charges are expected to include war crimes and crimes against humanity.
The BBC's Caroline Hawley in Baghdad says it was a small step on what will be a long and complicated legal path, but on Wednesday the first move was made to bring Saddam Hussein to justice.
The man who brutalised his country for decades said "good morning" as he entered the room at a secret location to be informed of his rights, she reports.
Salam Chalabi, director of the Iraqi Special Tribunal, explained to Saddam Hussein what would happen.
"He was nervous, very nervous, because he did not know what was happening," Mr Chalabi told US ABC television.
He said Saddam Hussein no longer had the bushy beard he was seen with in a video after his capture in December last year, and was slimmer.
"We first saw Saddam Hussein and he had lost weight. He was not the towering figure that one used to see on TV before.
Mr Chalabi said that after the proceedings, which lasted a few minutes, the former dictator was ordered "to leave the room".
Former Deputy Prime Minister Tariq Aziz was among the 11 other defendants who appeared before the judge.
Mr Chalabi also said another of the defendants, Ali Hasan al-Majid, known as "Chemical Ali" for his role in poison gas attacks, appeared nervous.
"He looked very scared. He was shaking," Mr Chalabi said.
A spokesman for UK Prime Minister Tony Blair welcomed the planned openness for the trials of the leaders of the former regime.
"We have made clear our opposition to the death penalty but Iraq does now have a sovereign government and we must respect that the sovereign Iraqi government is taking charge of governing the country," he said.
US hand forced
Saddam Hussein, who was captured by US forces near his home town of Tikrit in December, is unlikely to face a full trial for several months.
"These people... will face justice before the special
Iraqi court created in January to try members of the former regime for crimes against humanity, genocide and war crimes," said interim Prime Minister Iyad Allawi.
The transfer is a symbol of Allawi's new authority
The BBC's John Simpson in Baghdad says the interim prime minister has forced the Americans' hand by insisting on the transfer of Saddam Hussein.
Washington had reacted frostily to the idea when it was first suggested earlier this month, he says, but could not refuse without doing serious damage to Mr Allawi's authority.
The International Red Cross said the US would have been breaching the Geneva Convention if it had continued to hold him after the new government had taken over.
'Court of vengeance'
Saddam Hussein is expected to face war crimes charges over the 1990 invasion of Kuwait and subsequent suppression of Shia and Kurdish uprisings, the use of chemical weapons against the Kurds in 1988 and the 1980-1988 Iran-Iraq war.
If convicted, he could face the death penalty.
Tariq Aziz - Deputy PM
Taha Yassin Ramadan - Vice-President
Sultan Hashim Ahmad al-Tal - Defence Minister
Ali Hasan al-Majid - "Chemical Ali"
Watban Ibrahim Hassan al-Tikriti - Saddam Hussein's half-brother - intelligence minister
A member of Saddam Hussein's legal team, Giovanni di Stephano, told the BBC they would prepare his defence once the charges against him were made clear.
The lawyer demanded access to his client and an independent medical assessment to see if he was fit to stand trial.
Another lawyer acting for the former president, Emmanuel Ludot, said that Saddam Hussein would refuse to recognise the legality of the court.
"It will be a court of vengeance, a settling of scores," Mr Ludot said.