Seated next to Afghanistan's former King Mohammed Zaher Shah in a palace at the Foreign Ministry, Karzai signed a decree formally declaring the 162-article document ratified earlier this month as the country's supreme law.
"Congratulations!" he called to Afghan leaders who helped draw up the new charter, as Cabinet ministers and foreign diplomats applauded at the brief ceremony.
The step was just the latest under a U.N.-sponsored peace drive designed to rebuild the Afghan state since a U.S.-led invasion drove out the Taliban two years ago.
The constitution outlines a tolerant, democratic Islamic state under a strong presidency — as sought by Karzai — a two-chamber parliament and an independent judiciary.
Ratified Jan. 4 after a sometimes bruising debate at a 500-member loya jirga, or grand council of representatives from across the country, the text also declares men and women equal before the law — a victory for human rights advocates.
Karzai has praised the new constitution, which also recognizes minority languages while giving few powers to provincial authorities, as a chance to pull the country together after nearly a quarter-century of violence.
But celebrations of its adoption have been tempered by a fresh wave of attacks across the south and east of the country blamed on holdouts from the hardline Islamic Taliban and their anti-government allies.
About 60 people have been killed in violence in the past three weeks — including 15 civilians, most of them children, who died in a Jan. 6 bombing in the southern city of Kandahar.
The United Nations has warned that countrywide elections to be held under the new constitution in June may have to be delayed because of poor security, and can only go ahead at all if the situation improves.
So far, only about 500,000 of the estimated 10 million Afghans eligible to vote have been registered, and U.N. teams have yet to venture into the riskiest parts of the country.
Karzai is widely expected to win the presidential vote.
By Stephen Graham