The 82-year-old reportedly suffered a heart attack. He is said to be receiving medical treatment in the Red Sea resort of Sharm el-Sheikh, where he has been in internal exile since a mass uprising toppled him earlier this year.

Egypt's interim government issued subpoenas to the former leader and his sons over the weekend, compelling them to testify in court over claims they illicitly acquired wealth and abused their power during Mubarak's three-decade reign over the Arab world's most populous nation.

News of Mubarak's admittance to hospital emerged just hours after the prime minister, Essam Sharaf, used a live TV address to vow that "no one is above the law". Pro-reform protesters have made the prosecution of Mubarak a cornerstone of their demands in the weeks following the president's resignation.

As rumours circulated regarding both Mubarak's status and whereabouts, armed civilians – backed by military police – stormed Cairo's Tahrir Square, where a hardcore minority of protesters remain camped out. Several arrests were made.

Activists, who were already critical of Egypt's military authorities over their handling of the post-Mubarak "transition period", expressed scepticism and anger at news of his admittance to hospital.

"The fallen dictator must be locked up in a prison cell, not placed in a five-star hospital," argued Jano Charbel, a journalist. "I am afraid Mubarak will give us the biggest slap [by dying]," said the blogger Zeinobia.

The health prospects of Mubarak, who has long been ill, could have a major impact on the volatile internal politics of Egypt, where divisions between pro-reform protesters and the interim authorities – which are accused by some of being too slow in holding those in the Mubarak regime to account and insincere in their efforts to build democratic institutions – are threatening to bubble over.

On Friday, a rally in Tahrir ended in bloodshed after some army officers joined in demonstrations against the country's ruling generals, prompting military police to attack the crowds.

That incident raised suspicion among the armed forces, with the distrust deepening on Sunday when Mubarak – who has been under house arrest – was allowed to make his first public statement since being forced from office. He used the pre-recorded speech to deny accusations of embezzlement, saying: "They aim to tarnish my reputation and discredit my integrity, my stance, my political and military history during which I worked hard for Egypt and its people in peace and war."

Egypt's interim prime minister moved swiftly to try to defuse the growing split between demonstrators and army chiefs, who are effectively running the country by decree until elections are held later this year.

Essam Sharaf used a TV address to apologise for the violence in the capital, and promised a judicial investigation into the events. "What happened is strange, because Tahrir is the place where the people and the army became one," he told viewers, echoing a popular chant during the uprising.

On Monday, the ex-general secretary of Mubarak's NDP party, Safwat el-Sherif, was arrested. Sherif, a one-time giant of Egyptian politics, is being held for 15 days on charges of misusing public funds.