KHARTOUM, Sept. 26 (Xinhua) -- A conference, to be hosted by the Sudan People's Liberation Movement (SPLM), is scheduled to begin Saturday in Juba, southern Sudan, with the participation of 25 Sudanese opposition parties.

The conference tends to set up a map for tackling Sudan's internal and external issues.

Leaders of the National Umma Party, the Popular Congress Party, the Communist Parties, Umma Party (reform and renewal), the Arab Ba'ath Socialist Party and others will take part in the conference besides the SPLM.

In the meantime, 40 Sudanese political parties, including the ruling National Congress Party (NCP) Thursday announced their boycott of the conference, saying the conference "had hidden agenda."

The four-day conference will discuss many political issues including the 2010 general elections, the 2011 referendum in southern Sudan, the Darfur conflict and the future of the relationship between the NCP and the SPLM.

The SPLM said the conference did not tend to establish an opposition alliance against its NCP partner.

"This conference does not tend to form alliances because alliances are usually established against enemies," Malik Aqar, a leading SPLM member said in a press statement.

"We meet to form a unified front to discuss these important issues and we seek to reach a consensus on them," he added.

According to political observers, the Juba conference is aimed at establishing an alliance before the forthcoming general elections in April next year and that the leaders of the attending parties could agree on a presidential candidate to face the Sudanese President Omar al-Bashir, the sole NCP candidate.

The Sudanese Media Center (SMC) reported that prominent leading SPLM members are pushing towards the establishment of an alliance with the opposition parties to select a candidate to compete with the NCP candidate and with a full support of the SPLM.

The SMC quoted Hassan al-Saoury, the chairperson of the Sudanese Political Society, describing the conference as "a form of pressure practiced by the opposition and the SPLM on the NCP," saying that any alliances between the SPLM and the opposition parties might not work due to differences in their stances and as these parties lack popular support.

The NCP has refused to participate in the Juba conference. "We set our terms but there was no response and therefore we decided to boycott it," the party's political secretary Mandour al-Mahdi was quoted in a press conference in Khartoum Thursday as saying.

The NCP demanded to be involved in the arrangements for the conference together with all the registered Sudanese political parties which are about 60.

Convening of the Juba conference and the NCP's rejecting stance constitute a new dimension in the tensed relationship between the two signatories to the Comprehensive Peace Agreement (CPA) which ended a two-decade war between northern and southern Sudan in 2005.

The two sides have their differences over the implementation of some items of the CPA, namely with regard to the referendum on unity or separation between north and south, which is scheduled for January 2011.

The SPLM and the NCP have not yet agreed on an regulating law to the referendum, whereas the NCP insists that all southerners, whether they reside in the south, north or outside Sudan, have the right to participate in the referendum, while the SPLM says only the southerners who live in the south have that right.