TRIPOLI (Reuters) - The leader of a political group in Libya's oil-rich Cyrenaica province declared it a self-governing region in a speech on Saturday, evidence of growing pressure for a planned new constitution to adopt a federal structure.

Ahmed Zubair al-Senussi, head of the Cyrenaica Council and a distant relative of Libya's last king, has no formal authority under current transitional arrangements and it is not clear how he would be able to implement the declaration on the ground.

His council, however, is representative of factions in the east that have become more unified by concerns over how to establish the rule of law in a country awash with weapons and rival militias two years after the end of the war to depose Muammar Gaddafi.

The national assembly in Tripoli said it would form a committee to examine the declaration by Senussi, which included promises to form a new parliament and security force for the region.

"As of today, Cyrenaica is a federal region within the framework of the Libyan state," Senussi said in a speech to a rally in the eastern town of al-Marj. "Cyrenaica will activate the 1951 constitution and establish a parliament."

Calls for federal rule have become more vocal since Gaddafi's overthrow in 2011, fuelled by familiar complaints in the east of the country that it has not been given a fair share of Libya's wealth and the weakness of the central government.

But Senussi gave no timeline for implementing his proposals and called for the Tripoli government, which is leading the nation as it prepares to write a new constitution, to transfer money to the region.

For about 10 years after it became an independent state in 1951, Libya was run along federal lines, with power devolved to Cyrenaica, the southern province of Fezzan, and Tripolitania in the west of the country.

Libya centralized its government in the last years of the rule of King Idris, and Gaddafi accelerated the process when he came to power in a military coup in 1969.

The eastern city of Benghazi was the cradle of the 2011 uprising but residents now complain of Tripoli's inability to provide security by disarming militias or moving towards writing a constitution.

(Reporting by Ghaith Shennib; additional reporting by Firas Bosalum in Benghazi; Writing by Marie-Louise Gumuchian; editing by Patrick Graham)