Tens of thousands of Kenyans gathered to celebrate the country’s president Mwai Kibaki signing into law a new constitution promised more than two decades ago.

The new US-style laws include a Bill of Rights and reforms to policies designed to address long-held grievances over land stolen by corrupt politicians.

Demands for the constitution stretch back to the end of one-party rule by former president Daniel Arap Moi at the beginning of the 1990s.

Two-thirds of Kenyans voted to approve the new document in a referendum held earlier this month....