* Saif al-Islam says new constitution being prepared
* Says situation changing in Tripoli's favour
CAIRO, April 20 (Reuters) - The son of Libyan leader Muammar Gaddafi said the government would prevail over rebels trying to oust his father and a new constitution was ready for when the insurgency was defeated, Al Arabiya TV reported on Wednesday.
The Dubai-based satellite channel said Saif al-Islam, speaking on Libyan state television, accused the Benghazi-based rebel national council as being motivated by "power and oil wealth".
"Libya will not go back to what it was," Al Arabiya quoted Saif al-Islam as saying. "The era of the first Jamahiriya (people's republic) is gone and a new draft constitution has been prepared".
Saif al-Islam has the highest profile among Gaddafi's sons although, like his father, he has no official government position.
The rebels have been trying since mid-February to end Gaddafi's 41-year-old rule but have struggled against his more experienced and better equipped forces.
Despite NATO air strikes mandated under a U.N. Security Council resolution to protect civilians, they have made little advance towards Gaddafi's stronghold of Tripoli.
Saif al-Islam said the situation was changing in favour of the Libyan system, Al Arabiya reported. He said his father "will prevail" against the rebels.
The report gave no further details.