* Second stage of referendum to be held on Saturday
* Islamists say constitution vital to move forward
* Opposition say aspirations of all Egyptians not met
* Parliamentary election to follow if constitution passed
By Shaimaa Fayed and Tamim Elyan
CAIRO, Dec 16 (Reuters) - Egyptians voted in favour of a constitution shaped by Islamists but opposed by other groups who fear it will divide the Arab world's biggest nation, officials in rival camps said on Sunday after the first round of a two-stage referendum.
Next week's second round is likely to give another "yes" vote as it includes districts seen as more sympathetic towards Islamists, analysts say, meaning the constitution would be approved.
But the narrow win so far gives Islamist President Mohamed Mursi only limited grounds for celebration by showing the wide rifts in a country where he needs to build a consensus for tough economic reforms.
The Muslim Brotherhood's party, which propelled Mursi to office in a June election, said 56.5 percent backed the text. Official results are not expected until after the next round.
While an opposition official conceded the "yes" camp appeared to have won the first round, the opposition National Salvation Front said in a statement that voting abuses meant a rerun was needed - although it did not explicitly challenge the Brotherhood's vote tally.
Rights groups reported abuses such as polling stations opening late, officials telling people how to vote and bribery. They also criticised widespread religious campaigning which portrayed "no" voters as heretics.
A joint statement by seven human rights groups urged the referendum's organisers "to avoid these mistakes in the second stage of the referendum and to restage the first phase again".
Mursi and his backers say the constitution is vital to move Egypt's democratic transition forward. Opponents say the basic law is too Islamist and tramples on minority rights, including those of Christians who make up 10 percent of the population.
The build-up to Saturday's vote was marred by deadly protests. Demonstrations erupted when Mursi awarded himself extra powers on Nov. 22 and then fast-tracked the constitution through an assembly dominated by his Islamist allies.
However, the vote passed off calmly with long queues in Cairo and several other places, though unofficial tallies indicated turnout was around a third of the 26 million people eligible to vote this time. The vote was staggered because many judges needed to oversee polling staged a boycott in protest.
The opposition had said the vote should not have been held given the violent protests. Foreign governments are watching closely how the Islamists, long viewed warily in the West, handle themselves in power.
"It's wrong to have a vote or referendum with the country in the state it is - blood and killings, and no security," said Emad Sobhy, a voter who lives in Cairo. "Holding a referendum with the country as it is cannot give you a proper result."
As polls closed, Islamists attacked the offices of the newspaper of the liberal Wafd party, part of the opposition National Salvation Front coalition that pushed for a "no" vote.
"The referendum was 56.5 percent for the 'yes' vote," a senior official in the Brotherhood's Freedom and Justice Party operations room set up to monitor voting told Reuters.
The Brotherhood and its party had representatives at polling stations across the 10 areas, including Cairo, in this round. The official, who asked not to be identified, said the tally was based on counts from more than 99 percent of polling stations.
"The nation is increasingly divided and the pillars of state are swaying," opposition politician Mohamed ElBaradei wrote on Twitter. "Poverty and illiteracy are fertile grounds for trading with religion. The level of awareness is rising fast."
One opposition official also told Reuters the vote appeared to have gone in favour of Islamists who backed the constitution.
The opposition initially said its exit polls indicated the "no" camp would win comfortably, but officials changed tack during the night. One opposition official said in the early hours of Sunday that it would be "very close".
A narrow loss could still hearten leftists, socialists, Christians and more liberal-minded Muslims who make up the disparate opposition, which has been beaten in two elections since Hosni Mubarak was overthrown last year.
They were drawn together to oppose what they saw as a power grab by Mursi as he pushed through the constitution. The National Salvation Front includes prominent figures such as ElBaradei, former Arab League chief Amr Moussa and firebrand leftist Hamdeen Sabahy.
If the constitution is approved, a parliamentary election will follow early next year.
Analysts question whether the opposition group will keep together until the parliamentary election. The Islamist-dominated lower house of parliament elected earlier this year was dissolved based on a court order in June.
Violence in Cairo and other cities has plagued the run-up to the referendum. At least eight people were killed when rival camps clashed during demonstrations outside the presidential palace earlier this month.
In order to pass, the constitution must be approved by more than 50 percent of those casting ballots. There are 51 million eligible voters in the nation of 83 million.
Islamists have been counting on their disciplined ranks of supporters and on Egyptians desperate for an end to turmoil that has hammered the economy and sent Egypt's pound to eight-year lows against the dollar.
The army deployed about 120,000 troops and 6,000 tanks and armoured vehicles to protect polling stations and other government buildings. While the military backed Mubarak and his predecessors, it has not intervened in the present crisis.