Kenya has adopted a new constitution, more than three weeks after it was overwhelmingly approved in a national referendum. Tens of thousands of people watched as President Mwai Kibaki signed the document into law at a large ceremony in the capital, Nairobi. The debate over a new constitution has lasted 20 years. Sudanese President Omar al-Bashir was present at the event, despite being wanted for war crimes. Human Rights Watch earlier called on the Kenyan authorities to either "arrest him or bar him entry" if he were to attend. Kenya has ratified the statute requiring it to co-operate with the International Criminal Court, which has issued an arrest warrant for Mr Bashir. However, last month the African Union instructed its members - which include Kenya - not to apprehend Mr Bashir. 'Huge cheer' The constitution is expected to bring significant changes. Some have billed it as the most important political event in Kenya's history since it gained independence from Britain in 1963. The large crowd gathered in Nairobi's main Uhuru park to watch their leader promulgate the new document, amid gun salutes and a grand parade. After Mr Kibaki signed his name, he held the document up and there was a huge cheer from the audience. The new constitution will bring a more decentralised political system, which will limit the president's powers and replace corrupt provincial governments with local counties. It will also create a second chamber of parliament - the Senate - and set up a land commission to settle ownership disputes and review past abuses. It is hoped that the changes will help bring an end to the tribal differences that have brought violence to the country in the past. 'Optimism' The BBC's East Africa correspondent, Peter Greste, says the debate for a new constitution ebbed and flowed with each new political crisis until the elections of 2007, which were followed by the worst ethnic violence Kenya has yet seen. In the wake of the violence, everyone acknowledged that something fundamental had to change if the country was to avoid yet more trouble, our correspondent says. "The historic journey that we began over 20 years ago is now coming to a happy end," Mr Kibaki said earlier this month after the results of the referendum were announced on 5 August. "There will be challenges along the way. But it is important that we look forward with renewed optimism to better days ahead." Our correspondent says that the previous constitution allowed politicians to exploit tribal divisions, left courts weak, and concentrated power in the president's hands. While many Kenyans say that this is just a start - and that things could still go very wrong - most believe it is a fundamentally better document than the last. President Kibaki won a landslide victory in 2002 promising to change the constitution within 100 days of taking office. In 2005, he held a referendum but it failed to pass. The previous constitution was negotiated with the British in the early 1960s. Source: BBC