"These demands are shared by the other opposition parties with a few variations," Nugdalla, whose party won the last legislative elections in 1986, said.
"If these conditions are not fulfilled by April 6, the Umma party will boycott all the process of elections."
Nugdalla said "it is not too late to save" the country's first multiparty polls in 24 years.
However, there has been no comment from any of the other opposition parties about whether they might be willing to reconsider their participation.
The Sudan People's Liberation Movement (SPLM), a southern movement which is in coalition with the National Congress Party (NCP) of Omar al-Bashir, Sudan's president, said on Thursday that Yasir Arman, its candidate, would not standbecause of electoral irregularities.
Arman was seen as the main challenger to al-Bashir, who is now almost certain to win the vote.
The Democratic Unionist Party (DUP) has also said it would pull out of the presidential vote, but would take part in parliamentary and regional polls.
"I want to confirm that we will participate in the elections at all levels except for president," Salah al-Basha, the DUP spokesman, said.
The US special envoy to Sudan said after meeting officials of the National Election Commission (NEC) on Saturday that the election would be as "free and fair as possible".
"They [electoral commission members] have given me confidence that the elections will start on time," said Scott Gration.
"These people have gone to great lengths to ensure that the people of Sudan will have access to polling places and that the procedures and processes will ensure transparency."
Gration's meeting with the NEC came after the US, which has invested heavily in the elections required under a 2005 peace deal it mediated between north and south, said opposition leaders had raised "legitimate concerns".
"We certainly hope the parties can reach agreement so there will be maximum participation," PJ Crowley, the US state department spokesman, said.
"What's important here is to put together credible, legitimate institutions of government that can govern all of Sudan."
Asked if a delay would be appropriate, Crowley said: "At the present time, we're working hard to try to resolve these issues. I think we are still aiming for the election to occur on April 11."
Ibrahim Ghandour, a senior NCP official, said the opposition threats would not affect the poll's legitimacy.
"If you know beforehand that this is a lost contest, why waste your time and resources on a lost bet?" he said.
"This does not affect the legitimacy of the contest."
However, international observers and rights groups have said that all signs point to a flawed electoral process where the NEC is unlikely to deliver a free and fair election.
"The international community and the government of Sudan tried to build the elections like a house without foundation, and it is no surprise that [it] is falling apart pretty badly at this point," John Norris, the executive director of the Enough Project at the Center for American Progress, said.
Al-Bashir, who seized power in a coup in 1989, has threatened to expel foreign election observersdemanding electoral reforms.
Last week, he said that he would cancel a referendumscheduled for next January on southern independence if the SPLM boycott the election.