Former Sudanese Prime Minister and opposition leader Sadiq al-Mahdi has taken a swipe at the country's new constitution, branding it "deficient".
He accused the charter of being a bilateral agreement between the ruling National Congress and the Sudan People Liberation Movement (SPLM).
Other opposition parties were under-represented, he argued.
The constitution, brokered as part of a recent peace deal which ended a long civil war, came into force on Saturday.
"We think it is a deficient draft and a deficient constitution," Mr Mahdi said.
"There are many problems now in the east, in the west and in other parts of the Sudan whose resolution will require them to be represented in a comprehensive agreement and be represented in the constitution."
'Diarchy'
Under the new constitution, 52% of government and parliament posts will be held by the National Congress led by President Omar el-Bashir, while former rebel leader John Garang's SPLM has been given 28% of power.
Northern and southern opposition parties share the remaining 20%.
Mr al-Mahdi acknowledged the constitution was "a step forward", but warned that the current agreement would lead to a "diarchy" - a country headed by two leaders.
He called on all those who had been left out to unite in a national front which would monitor "oppression and corruption" in the country.
Mr Garang was sworn in as Sudan's vice-president on Saturday after the signing of the new charter during a ceremony in Khartoum.
Under the peace deal, Sudan's new oil wealth will be shared between north and south, Islamic Sharia law will not be applied in the south and the south will hold a referendum on secession in six years' time.
The conflict between the predominantly Muslim north and Christian south, which lasted more than two decades, claimed 1.5m lives.