ALEXANDRIA, Egypt — Christians clashed with Egyptian police in the northern city of Alexandria on Saturday, furious over an apparent suicide bombing against worshippers leaving a New Year’s Mass at a church that killed at least 21 people. It was the worst violence against the country’s Christian minority in a decade.

The Interior Ministry blamed “foreign elements,” and the Alexandria governor accused al-Qaida, pointing to the terror network’s branch in Iraq, which has carried out a string of attacks on Christians there and has threatened Egypt’s Coptic Orthodox Christian community as well.

Egypt’s government has long insisted that the terror network does not have a significant presence in the country, and it has never been conclusively linked to any attacks here. If al-Qaida was involved, it raises the prospect of a serious new security threat within Egypt.

President Barack Obama condemned “this barbaric and heinous act” and said those behind it must be brought to justice.

The bombing, about a half hour after the stroke of the New Year, stoked tensions that have grown in recent years between Egypt’s Christians and the Muslim majority.

It was dramatically different from past attacks on Christians, which included shootings but not serious bombings, much less suicide attacks. Christians have increasingly blamed the government for not taking violence against them or anti-Christian sentiment among Muslim hard-liners seriously.