Four members of Sudan's opposition Popular Congress Party have been released under a presidential amnesty after more than eight years in prison, the party said on Thursday.

"They have been freed today after spending 103 months in prison," said Kamal Omer, the chief of political affairs for the party led by Sudan's veteran Islamist Hassan al-Turabi.

He said the four -- Mousa Eshag, Ibrahim Adam, Ali Ahmed and Abdelhalim Adam -- had been sentenced in 2003 to 15 years in prison over a "coup" plot.

President Omar al-Bashir on April 1 announced that all political prisoners would taste freedom as the government seeks a broad political dialogue, "including (with) those who are armed".

Thursday's release brings to 11 the number of political detainees known to have been freed so far.

Omer called the news bittersweet because a fifth person also imprisoned for the 2003 incident is still held, along with detainees from Sudan's current war zones of South Kordofan, Blue Nile and Darfur.

Farouk Abu Issa, who heads the opposition coalition of more than 20 parties including Popular Congress, has said hundreds of political prisoners are still detained.

The four men released on Thursday had actually walked out of prison two months ago following a separate release order by Bashir, but members of the powerful National Security and Intelligence Service rearrested them, Omer said.

Seven other political inmates last week became the first to be released under Bashir's April 1 amnesty.

Most were opposition members held since early this year for their connection with a conference in Kampala, Uganda, which led to a charter for toppling Bashir's 23-year government.

In the charter, opposition parties and Sudanese rebel groups agreed on removing the regime using both armed and peaceful means.

Bashir's speech elaborated on an offer made by Vice President Ali Osman Taha, who invited the rebel Sudan People's Liberation Movement-North (SPLM-N) and opposition political parties to join a constitutional dialogue.

Sudan needs a new constitution to replace the 2005 document based on a peace agreement which ended a 23-year civil war and led to South Sudan's separation in July 2011.

The government had long rejected negotiations with the SPLM-N, which is fighting in South Kordofan and Blue Nile, and which it alleged was backed by South Sudan.

A potential way forwards emerged in early March after Sudan and South Sudan finally settled on detailed timetables to implement crucial economic and security pacts to ease tensions.