Mustafa Abdul-Jalil also told thousands of supporters at a ceremony that Islamic Sharia law would be the "basic source" of legislation in the country and that existing laws that contradict the teachings of Islam would be nullified. "This revolution was looked after by God to achieve victory," he told the crowd.
In an address that set an Islamist tone for post-Gaddafi Libya, he said new banks would be set up to follow the Islamic banking system, which bans charging interest.
Mr Abdul-Jalil called on Libyans to show "patience, honesty and tolerance" and eschew hatred as they embark on rebuilding the country at the end of an 8-month civil war.
In a gesture that showed his own piety, he urged Libyans not to express their joy by firing in the air, but rather to chant "Allahu Akbar," or God is Great. He then stepped aside and knelt to offer a brief prayer of thanks.
"This revolution was looked after by God to achieve victory," he told the crowd at the declaration ceremony in the eastern city of Benghazi, the birthplace of the uprising against Gaddafi began.
He thanked those who fell in the fight against Gaddafi's forces. "This revolution began peacefully to demand the minimum of legitimate rights, but it was met by excessive violence."
Mr Abdul-Jalil said new banks would be set up to follow the Islamic banking system, which bans charging interest. For the time being, he said interest would be cancelled from any personal loans already taken out less than 10,000 Libyan dinars (about £4,700).
He also announced that all military personnel and civilians who have taken part in the fight against Gaddafi would be promoted to the rank above their existing one. He said a package of perks would later be announced for all fighters.
"Thank You, thank you to the fighters who achieved victory, both civilians and military," he said. He also paid tribute to the Gulf Co-operation Council, a six-nation alliance led by Saudi Arabia, The Arab League and the European Union. Nato, which aided the anti-Gaddafi fighters with airstrikes, performed its task with "efficiency and professionalism."