Millions of Egyptians will vote on Saturday in the country's first democratic election since Hosni Mubarak was toppled as president after 18 days of street protests.

In a key test for the military authorities overseeing Egypt's transition to democracy, voters are being asked to support or reject a package of constitutional amendments drawn up by a special committee of legal experts.

After decades of elections that featured only a single candidate or were blatantly rigged, Saturday's referendum is the first ballot in modern Egyptian history in which the outcome is genuinely unknown. The landmark vote is also proving deeply divisive and comes at a critical juncture for the nation's revolution, with pro-change activists accusing the army of using brutality and torture in an attempt to shut down dissent, and the newly appointed prime minister warning that a counter revolution could destroy the state.

"Regardless of the outcome, this election will turn a new page in Egypt's history," said Nabil Abdel Fattah, a political analyst.

The proposed amendments include new term limits on the presidency, restrictions on the ability of the government to maintain emergency law, and heightened judicial supervision of elections. If they pass, Egypt will elect a new parliament and president within the next six months.

Critics believe the amendments do not go far enough and that a new constitution should be written from scratch. They accuse the armed forces of rushing the process of transition to benefit conservative political forces that are already well organised, such as the Muslim Brotherhood and the remnants of Mubarak's NDP party, leaving radical new youth movements that emerged from the revolution out in the cold.

Sixteen political parties and coalitions have called for a no vote, alongside several leading intellectuals and political heavyweights, including the secretary-general of the Arab League, Amr Moussa, and the former director general of the International Atomic Energy Agency, Mohamed ElBaradei. "Keeping Mubarak's constitution, even temporarily, is an insult to the revolution," said ElBaradei, who, like Moussa, is expected to run for president. "Voting yes in the referendum resuscitates Mubarak's constitution and will result in a flawed parliament."

ElBaradei's stance has been backed by a wide spectrum of senior lawyers and cultural icons, from the country's first female judge to bestselling novelist Alaa al Aswany. Many of them fear that a yes vote will legitimise the army's much-criticised path of transition and offer only cosmetic changes to the system built up under 30 years of Mubarak's rule, potentially allowing elements of the old regime to re-entrench themselves. Some claim that by endorsing an amended version of the 1971 constitution, Egyptians will be "legalising dictatorship", as the document concentrates power strongly in the hands of the presidency and offers few checks and balances within a top-down political system.

That message appears to be getting through to the 45 million eligible voters; a recent government-commissioned opinion poll suggests that 59% are poised to reject the reform package. Such an outcome would be a surprising rebuke to a military establishment unaccustomed to public hostility, but according to Amr Hamzawy, a professor of political science at Cairo University, widespread scepticism at the military's model of transition is understandable.

"These amendments reflect the sort of debate that was going on over the past few years about how best to push Egypt towards a political opening," he said. "But after the revolution it's no longer a question of how to find a small political opening, it's a question of how to create a completely new system, and therefore this package does not live up to the people's expectations."

The Muslim Brotherhood, Egypt's largest organised movement, is campaigning for a yes vote, as are some Salafist groups and the new incarnation of Mubarak's NDP. Although some have portrayed the referendum as a battle pitting secularists against Islamists and young revolutionaries against the political old guard, the reality is more complex.

The issue has driven a wedge between activists who led the occupation of Tahrir Square, with some arguing that passing the amendments and holding quick elections is the best way of swiftly pushing the military out of the political arena and forming a civilian government that can tackle the many social and economic demands – like improved rights for women and a higher minimum wage – that, beyond the fall of Mubarak, lay at the heart of the revolution.

"What we're seeing now is a major split within the revolution," Alaa Abd el Fattah, a prominent leftist activist and supporter of the yes vote, said. "There is a socio-economic class divide at work here. At a certain point after Mubarak stepped down, a middle class discourse began to dominate, one that cast the ongoing struggle by workers and other groups for their rights as narrow and self-interested and not part of the revolution.

"Now they say the revolution is being stolen through this referendum. Which part of the revolution is being stolen? A new constitution, from my perspective, was never on the agenda, but reform of public institutions and the implementation of social and economic demands were much more of a core part of what we were fighting for. If you claim to care about workers' rights then you need a parliament now that can legislate to provide those rights."

The referendum comes on the back of a tumultuous fortnight in Egypt, which saw several outbreaks of sectarian violence, the ransacking and then dismantling of the country's hated state security apparatus, and a tense standoff between pro-change protesters and soldiers that ended in many activists being forcefully cleared from Tahrir Square and taken into detention, where rights groups say they are being tortured.

Numerous demonstrators have given public accounts of being arrested and held inside the Egyptian Museum, where they were beaten and electrocuted by military officers. "There can be no break from the abuses of the past while security forces – including military personnel– abuse people with impunity," said Joe Stork, deputy Middle East and north Africa director at Human Rights Watch. "These brutal methods of abuse were the hallmarks of President Mubarak's rule, and protesters sat in the square for 40 days so that they would end."

The army has declared that no strikes or protests will be tolerated on the day of the referendum, and have imposed a two-day domestic media blackout on the event to offer voters a suitable "democratic atmosphere" in which to cast their vote.