Islamists attacked the opposition Wafd party's newspaper offices in central Cairo with petrol bombs and birdshot on Saturday, security sources said.
The violence flared as Egyptians voted in a referendum on a new constitution intended to pull the country out a growing political crisis.
Two people were injured and firefighters said they had put out the flames, a Reuters witness said.
Mostafa Shafik, managing editor at Wafd's newspaper, which is located next to the party headquarters, said the paper's offices had been destroyed.
"I can see from the window that cars are damaged, while the headquarters of the newspaper are destroyed," he told Reuters.
He said police had stood by when the attack was taking place "and at the end fired tear gas at both us and the attackers".
Shafik added: "The attackers used Molotov cocktails to enter, which left minor areas burned."
A Reuters photographer saw a dozen or so cars damaged inside the Wafd headquarters' grounds, their windows broken. Glass was also broken in the headquarters, but he saw no immediate signs of fire damage.
The run-up to the referendum has been marred by violence in Cairo and other cities. Several party buildings belonging to the Muslim Brotherhood's Freedom and Justice party have been burned in protests across the country.
referendum is on a divisive draft basic law that has pitted Islamist
supporters of President Mohamed Morsi against a liberal, secular and
Christian opposition in often bloody clashes in Cairo and other cities.
opposition says the constitution is too Islamist and tramples on
minority rights. Morsi's supporters say the charter is needed if
progress is to be made towards democracy nearly two years after the fall
of military strongman Hosni Mubarak.
Mere hours after the polls
opened on Saturday, a man was arrested for shooting in the air outside a
polling station in Gharbiya in the first sign of violence since the
voting started, Al-Masry Al-Youm reported.
Egyptian opposition, the National Salvation Front, reportedly documented over 50 violations at voting sites across the country.
The opposition claimed that a judge overseeing voting in the referendum directed voters to vote "yes" on the constitution, Al-Masry Al-Youm reported.
reports indicated that supporters of the Muslim Brotherhood were
campaigning in the vicinity of the polling stations by distributing
flyers promoting voting "yes" on the constitution, according to Al-Masry Al-Youm.
Alexandria on Friday, tensions boiled over into a street brawl between
rival factions armed with clubs, knives and swords. Several cars were
set on fire and a Muslim preacher who had urged people to vote "yes" to
the constitution was trapped inside his mosque by angry opposition
In the capital, Cairo, both sides made low-key final efforts to rally supporters.
Islamists gathered peacefully at one of the main mosques, some shouting
"Islam, Islam" and "We've come here to say 'yes' to the constitution".
Opposition supporters - who have been urged to vote "no" by their leaders - assembled outside the presidential palace.
building remains ringed with police, soldiers and tanks after street
clashes caused at least eight deaths earlier this month in violence
prompted by Morsi's decision to award himself sweeping powers in order
to ram through the new charter.
As darkness fell, there appeared
to be more bystanders and street vendors present than opposition
demonstrators. A woman addressed the crowd through a loudhailer,
shouting obscenities about Morsi, but many in her audience seemed more
interested in drinking tea or having their picture taken in front of a
"I don't like Morsi," said Moustafa Ahmed, 25, a teacher.
"But I haven't decided what to vote for tomorrow so I decided to come
here to listen to the protesters and chat to them one last time."
Two days of voting
referendum will be held on two days - this Saturday and next - because
there are not enough judges willing to monitor all polling stations
after some in the judiciary said they would boycott the vote.
are being asked to accept or reject a constitution that must be in
place before a parliamentary election can be held next year - an event
many hope can steer the country towards stability.
The measure is
generally expected to pass, given the well-organized Muslim
Brotherhood's record of winning elections since the fall of Mubarak.
Many Egyptians, tired of turmoil, may simply fall in line and vote
If the constitution is voted down, a new assembly will
have to be formed to draft a revised version, a process that could take
up to nine months.
Just over half of Egypt's electorate of 51
million will vote in the first round in Cairo and other cities. Polling
stations open at 8 a.m. and close 12 hours later.
results will not be announced until after the second round, though it is
likely that details will emerge after the first round that will give an
idea of the overall trend.
To provide security for the vote, the
army has deployed about 120,000 troops and 6,000 tanks and armored
vehicles to protect polling stations and other government buildings
While the military backed Mubarak and his predecessors, it has not intervened on either side in the present crisis.
The charter has been criticized by some overseas bodies.
International Council of Jurists, a Geneva-based human rights group,
said it falls short of international standards on the accountability of
the armed forces, the independence of the judiciary, and recognition of
United Nations human rights experts said the draft
should be reviewed to ensure that Egypt meets its obligations under
international law on equality and women's rights.