CAIRO — Egypt’s leading opposition group urged its followers Wednesday to vote against an Islamist-inspired draft constitution, ending weeks of indecision over whether antigovernment forces should boycott the referendum, which begins this weekend and pits secularists against the Muslim Brotherhood.

The move by the National Salvation Front is a crucial test of its popularity against President Mohammed Morsi and his Islamist supporters. The opposition movement has revived the country’s revolutionary fervor but has been marred by divisions and poor organization, which are expected to be exploited by the Brotherhood’s vast grass roots networks.

The National Salvation Front “decided to call upon the people to go to the polling stations and reject the draft by saying ‘No,’ ” said Hamdeen Sabahi, a former presidential candidate and one of the group’s leaders.

Perhaps sensing a possible setback, he added: “The referendum is not the end of our journey.”

The Front said its decision was contingent upon judicial oversight, international monitors and increased security at polling stations. It is unclear whether those demands can be met. Many judges, angry at a recent power grab by Morsi that weakened the courts, have refused to supervise the referendum. That forced the Islamist leader to announce that voting would be held over two successive Saturdays — beginning this weekend — so participating judges can be rotated throughout the country.

A dialogue for national unity called for by the military between Morsi and different political factions was canceled Wednesday. The armed forces said few organizations agreed to attend. But the military, criticized for its harsh rule of the country after Hosni Mubarak was deposed last year, faced a backlash over perceptions it was meddling in politics months after it had handed power to Morsi.

A compromise appears increasingly elusive as the president pushes for the referendum and the opposition tries to rally voters against a document it fears will strengthen Islamists and jeopardize civil rights and freedom of expression.

That prospect was highlighted Wednesday when a court sentenced Albert Saber, an atheist from a Christian family, to three years in prison for insulting Islam. He was charged with posting Internet links to a video produced in California that denigrated the Prophet Muhammad and ignited protests in September.

The country’s latest crisis intensified in late November, when Morsi expanded his powers and freed his office from judicial oversight. The president has since offered concessions, but — despite clashes that have killed at least eight people — he has refused demands to postpone the referendum until a document they find more representative is written.

Egyptians living abroad began voting Wednesday at embassies on the proposed constitution.

The voting scheme has been so rushed that many voters don’t know where to cast their ballots.

Morsi has asked the army — once his archenemy — to protect public institutions until the voting is finished. Meanwhile, clerics from Al-Azhar University, the most revered institution in Sunni Islam, rallied not with those supporting Morsi but with leftists and liberals against the president.