NAIROBI, Kenya (AP) — African leaders joined tens of thousands of Kenyans on Friday to witness a once-in-a-generation event: the signing into law of Kenya’s new constitution.

The East African nation replaced a colonial-era constitution with one that curtails the president’s powers with an American-style system of checks and balances.

President Mwai Kibaki’s official signing into law of the new constitution has been billed as the single most important political event in Kenya’s history since it gained independence from Britain in 1963.

Friday’s event comes after an overwhelming majority of Kenyan voters adopted the new constitution in an Aug. 4 referendum. Kibaki’s signature formally marks the end of a decades-long struggle to cut down the massive powers of the presidency.

The government and parliament now must implement the ambitious document that requires, among other things, the formation of a Supreme Court and a Senate. It also demands that the country’s judiciary be vetted to rid it of corrupt or incompetent judges and that parliament pass 49 new laws.

Both Kibaki and Prime Minister Raila Odinga back the new charter, part of a reform package they committed themselves to after signing a power-sharing deal in February 2008. That deal ended violence that killed more than 1,000 people following Kenya’s disputed December 2007 presidential vote.

A surprise guest at Friday’s ceremony was Sudanese President Omar al-Bashir, who faces charges of genocide and crimes against humanity. This is the second time that al-Bashir has risked arrest by traveling to a member state of the International Criminal Court. In July he traveled to Chad.

Before his visit to Chad, al-Bashir had traveled abroad only to countries that are not ICC members since he was first charged in connection with violence in Sudan’s Darfur region in 2009. The ICC has no police force and depends on member states to enforce its orders.

It was not immediately known why Kenyan authorities allowed him to attend.

President Paul Kagame of Rwanda and President Yoweri Museveni of Uganda also attended the event.

Copyright 2010 The Associated Press.