The appeal by 40 parliamentarians was joined by 50 European and African human rights groups. They said in a separate letter that not acting on the crisis would mean turning "our back on the people of Darfur", where 200,000 people have been killed.

The U.N. security council approved in July a U.N.-African Union peace mission of 26,000 soldiers and police for Darfur. But U.N. peacekeeping chief Jean-Marie Guehenno has cast doubt on the mission due to restrictions imposed by Sudan.

"MPs, campaigners and human rights activists are all asking the same question: how can our leaders ignore one of the world's worst crises?" asked Glenys Kinnock, a member of the European Parliament.

"Especially when (Sudanese) President (Omar Hassan al-) Bashir, the man primarily responsible for so much of the suffering, is in their midst," she said in a statement.

More than 70 EU and African leaders meet on Friday evening for a banquet, followed by the summit on Saturday and Sunday.

The summit will cover peacekeeping, migration, human rights, energy, trade and development but there are no specific topics on the agenda. A Portuguese diplomat said any participant can raise any subject.

"There are no taboo subjects," the diplomat said.

The letter by parliamentarians said they were "surprised and disappointed" that no time had been allotted to discussing the Darfur conflict, which has displaced half a million people.

"The welfare of our citizens must be the first order of business whenever our leaders meet, and the devastating impact this ongoing conflict has on the lives of so many people should clearly place it near the top of the agenda of the EU-Africa summit," it said.

Human rights groups have already criticised Portugal for inviting Zimbabwean President Robert Mugabe, whom the West accuses of rigging elections and suppressing opposition. British Prime Minister Gordon Brown is boycotting the summit because of his attendance.

U.N. peacekeeping chief Guehenno has questioned the viability of a Darfur mission because of Sudanese restrictions on its movements and refusal to accept non-African troops.

U.N. Secretary-General Ban Ki-moon said on Thursday that a failure by countries to supply transport and attack helicopters had put the Darfur mission at risk.