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 decision came as the government forged ahead with its own plan, starting overseas voting in diplomatic missions for expatriates.

The move by the National Salvation Front is a key 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 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.

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 could be rotated around the country.

Zaghloul el-Balshi, head of the referendum's organizing committee, said that 9,000 judges had agreed to oversee the voting, though that could not be independently verified.

Egypt has nearly 13,000 polling stations, each of which normally requires a judge. Aides to Morsi have said judges are only needed to supervise the 9,000 main stations, while government employees or university lecturers can fill in at the rest.

The start of overseas voting showed Morsi's determination to go forward. The vote by half a million expatriates overseas could give hints about which way the referendum is going.

A dialogue for national unity called for by the military was canceled Wednesday. The armed forces said few agreed to attend.