Egyptian activists have threatened to bring mass pro-democracy protests back to Cairo, with a "million-strong" occupation of Tahrir Square planned for 8 July unless the ruling army generals abandon their current "roadmap" to democracy.

In an increasingly rancorous debate, which has developed into a proxy war between the nation's fledgling Islamist and secular political forces, 40 different liberal and leftist movements have joined forces to demand that plans to hold elections in September are dropped.

Campaigners fear the existing post-Mubarak transition programme – which would see September's ballot held under an amended version of Egypt's existing constitution and then allow members of parliament to oversee the writing of a new constitution – may cede permanent power to the Muslim Brotherhood and other religious groups, who are expected to dominate the poll.

Islamists have reacted furiously to the "constitution-first" campaign, arguing that it contradicts the results of a nationwide referendum held in March, in which 77% of the country backed a set of constitutional amendments and endorsed the idea that parliamentary elections should precede any new constitution.

"Egyptians will not remain silent on attempts by an irrelevant elite to impose a liberal secular constitution on the people," said the new Salafist party Al-Nour in a statement.

Egypt's interim prime minister, Essam Sharaf, has stoked controversy by suggesting elections could be delayed to allow the nation's "political landscape" to take shape – a key demand of many secular revolutionary groups who believe they have not had the time to develop their organisational capacity and are therefore likely to lose out in September to the Muslim Brotherhood, who are already a well-established presence in most towns and cities across the country.

But Sharaf – who took office 100 days ago this week and famously marked his appointment by entering Tahrir Square and announcing to protesters "I draw my legitimacy from you" – has faced a storm of criticism for his comments.

Any postponement of elections would contradict the official line held by Egypt's Supreme Council of Armed Forces, the country's de facto rulers until a civilian government is ready to take over. The prime minister later said his comments had been "misunderstood" and that the timetable for elections remained the same.

Disagreements over the timing and process of writing a constitution have become the main point of division within Egypt's febrile political landscape, which after decades of one-party rule is now being rapidly populated by a wide range of new forces.

"This is not just a debate about short-term political gain," said Egyptian journalist Ashraf Khalil, who has followed the issue closely. "Whoever wins the parliamentary elections is going to play a major role in writing the new constitution and they are therefore going to play a major role in shaping the political foundations of the new Egypt."

Concerns are mounting that a raft of new parties, including many claiming to represent the "revolutionary youth" that helped to overthrow Mubarak earlier this year, have not even completed the formal party registration process yet and will be in no position to mount a successful appeal for votes by September.

"The Brotherhood is clearly ready for elections now – it's been ready for 10 years – whereas the newer secular parties who could not participate in the political process under Mubarak are not," added Khalil.

"I'm not expecting an outright Brotherhood victory or the creation of some kind of Iranian theocratic state, but clearly if the new parliament does not have adequate representation from the movements that played such a key part in sparking the revolution then that's a cause for concern."

The Brotherhood's new political vehicle, the Freedom and Justice party, has vowed not to contest more than 50% of parliamentary seats and will not run a candidate for president in an effort to assuage concerns at home and abroad over a potential Islamist takeover of the state.

That has not been enough to reassure some critics, including a "national consensus conference" led by the deputy prime minister, which has called for the inclusion of an article in the new constitution that would task the military with "protecting" Egypt's civil institutions – a thinly-veiled warning against any potential attempt by an Islamist-controlled parliament to push for a religious state.

In an effort to secure a compromise, presidential hopeful Mohamed ElBaradei has proposed that elections precede the new constitution as originally planned, but that a new bill of rights should be drawn up beforehand that would supersede the constitution. This bill of rights would acknowledge the importance of Islamic sharia law in guiding legislation but also affirm Egypt's status as a civil state.

ElBaradei's plan has won some support, though questions remain about how such a legal document would be formulated.

"We have to search for a compromise," said political analyst Diaa Rashwan.

"We have already had a bitterly fought row over the constitutional amendments and the last thing we need now is to have another major battle between political forces that could destabilise things further."

Some activists though have labelled the entire row an unnecessary distraction from the more important job of formulating new policies that tackle Egypt's many socio-economic problems and improve the condition of over 30 million Egyptians still living below the poverty line. "This is a waste of time," tweeted the pro-change campaigner and former Google executive Wael Ghonim, who argued that the constitution debate had descended into an ideological point-scoring contest in which neither side was willing to back down.