Tens of thousands are protesting in the Egyptian capital Cairo against the decision by Hosni Mubarak's former spy chief to stand in presidential elections.
The protest in Tahrir Square, in central Cairo, was called by the Muslim Brotherhood and other Islamist groups.
They want Egypt's military rulers to approve a law banning former top Mubarak-era officials from standing.
The organisers fear Omar Suleiman and others seek to restore the old regime.
The former intelligence chief denies the accusation, saying his main aim is to bring back order and rein in the Islamists' growing power.
Show of force
Protesters at the rally, which is being held under the slogan "Defending the Revolution", chanted slogans against the former spy chief, as well as the ruling military council.
Many held banners bearing images of the crossed-out faces of Gen Suleiman and Ahmed Shafiq - a former prime minister who is also standing for president.
"If Omar Suleiman become president, everyone will take to the streets to protest. We have not got rid of Mubarak only to then bring in someone else who is like him," one protester, Adel Gamal, told the Associated Press.
The BBC's Jon Leyne in Cairo says the rally is a show of force by the Islamists, but adds that the numbers in Tahrir Square are nothing like those that turned against the regime last January.
Liberals and the youth activists who drove the 2011 revolution have said they will stay away from the protest.
Our correspondent says some protesters are linking Gen Suleiman to Israel. Some of their banners showed Gen Suleiman alongside the Star of David.
During his 18 years as the head of the powerful General Intelligence Department, he often acted as a go-between the two countries, and any links with Israel are deeply unpopular with Egyptians, our correspondent says.
Gen Suleiman announced his bid for the presidency in a surprise move on Friday.
Seen as one of the former president's closest confidants, he briefly became vice-president during the protests that toppled Mr Mubarak last year.
In an interview published on Monday, the former spy chief distanced himself from Mr Mubarak's regime, saying he objected to many of its policies.
He also said he wanted to prevent the Muslim Brotherhood from dominating Egypt, accusing it of "monopolistic practices".
Vying for power
The Muslim Brotherhood has warned of turmoil if Gen Suleiman wins the election.
It has put forward a candidate of its own, millionaire businessman Khairat al-Shater, despite earlier pledges not to contest the presidential elections.
Mr Shater has described Gen Suleiman's candidacy as an "insult to the revolution".
The Brotherhood-backed Freedom and Justice Party (FJP) became the largest party in parliament, in elections that were completed earlier this year.
On Thursday parliament passed a law banning former senior officials who served under President Mubarak from standing for political office.
The move was widely seen as being aimed against Mr Suleiman.
The measure still needs the approval of the ruling military council, which is made up of senior officers who served under the old regime.
It has been accused of seeking to shape the new Egypt according to its own interests.
The justice ministry described the new law as unconstitutional.