A second day of clashes with Egyptian police left two protesters dead in Cairo on Friday as anger against the ruling military boiled over amid fury at the deaths of 74 people in football-related violence.
Marchers took to the streets across the country to demand that the generals cede power immediately after a night of demonstrations in major cities left at least two more people dead elsewhere in Egypt.
In a sign of the growing threat the political turmoil poses to the economy, two female American tourists and their Egyptian tour guide were briefly abducted in the Sinai Peninsula, security officials said.
The two protesters in Cairo died of tear gas inhalation after being rushed to hospital unconscious from outside the interior ministry, medics said.
The health ministry said 544 people were injured in Friday's clashes.
Thick clouds of tear gas blanketed the road to the ministry, where protesters faced off with police after overnight clashes injured hundreds more, an AFP reporter said.
Demonstrators, many of them organised supporters of Cairo's Al-Ahly football club known as Ultras, held up a huge banner to the police that read: "Those who didn't deserve to die have died at the hands of those who don't deserve to live."
Many of the dead in Wednesday's football riot in the Mediterranean city of Port Said were thought to have been Al-Ahly supporters, set upon by partisans of the local Al-Masri side after the Cairo side lost 3-1.
The Ultras played a prominent role among anti-regime elements in the uprising that overthrew president Hosni Mubarak a year ago, and commentators and citizens have suggested pro-Mubarak forces were behind the massacre, or at least complicit.
Rocks and stones flew in all directions as police vans in Cairo repeatedly charged before retreating. At one point, police clubbed protesters who were just metres (yards) away from the ministry headquarters.
Across the street, a building housing the Tax Authority was on fire, state television reported without providing details.
A soldier injured outside the interior ministry building on Thursday died in hospital on Friday, the state MENA news agency reported.
In nearby Tahrir Square -- nerve centre of the mass rallies that forced Mubarak from power -- thousands chanted slogans against the military junta that took power when the president quit.
In the canal city of Suez, where two demonstrators were killed on Thursday, police fired birdshot and tear gas to disperse protesters, an AFP reporter said.
Under a volley of rocks and stones, ambulances ferried the injured out of the central Al-Arbaeen Square at the opposite end of the Suez Canal to Port Said where Wednesday night's stadium deaths enraged the nation.
Thousands also took to the streets to denounce the junta in Egypt's second city of Alexandria and in Port Said.
The Supreme Council of the Armed Forces (SCAF) blamed the unrest on "foreign and domestic hands targeting the country."
In a statement on Facebook, it urged "all political and national forces of this great nation to take a national and historic role and intervene ... to return stability."
Wednesday's clashes between fans of home team Al-Masry and Al-Ahly marked one of the deadliest incidents in football history, and came amid claims by witnesses that the security forces did little to prevent the rioting.
After the final whistle, victorious Al-Masry fans invaded the pitch, throwing rocks, bottles and fireworks at Al-Ahly supporters, causing panic as players and fans fled in all directions, witnesses said.
On Friday, Egypt's prosecutor general slapped a travel ban on the head of the Egyptian Football Association Samir Zaher -- a day after he was sacked -- and on ex-Port Said governor Mohammed Abdullah, who resigned following the clashes.
"This happened as security services stood by and did nothing, like they did in previous events, and perhaps they even contributed to the massacre," wrote Ibrahim Mansur, a columnist for the independent daily Al-Tahrir.
"This happened under the military council whose ouster the people are demanding, and who has proved that it is a failure," he said.
Egyptians have become increasingly angry with the junta, which they accuse of failing to manage the country and of human rights abuses.
For months, they have taken to the streets to demand the ouster of the SCAF and its chief, Field Marshal Hussein Tantawi, who was Mubarak's defence minister for two decades.
The SCAF has pledged to cede full powers to civilian rule when a president is elected by the end of June.
But widespread suspicions that the military aims to retain some powers after the transition were fuelled by comments from former US president Jimmy Carter after he met the generals repeatedly last month.
"When I met with military leaders, my impression was they want to have some special privilege in the government after the president is elected," Carter said.
In the Sinai, the abduction of the two US tourists by masked gunmen dealt a new blow to Egypt's already hard-hit tourism sector, despite their release unharmed several hours later.
The kidnappers stole watches, phones and money from the tour bus on its way from the historic St Catherine's monastery to the Red Sea resort of Sharm el-Sheikh before snatching the two women and their guide, security officials told AFP.