* Clashes between police, Brotherhood reported across Egypt
* Results of constitutional referendum expected Saturday
* Gunmen attack gas pipeline in central Sinai (Updates death toll)
CAIRO, Jan 17 (Reuters) - At least four people were killed as clashes between Muslim Brotherhood supporters and police flared up across Egypt on Friday, security sources said.
The violence erupted a day before Egyptian authorities are expected to announce official results of this week's referendum on a new constitution, part of an army-backed transition plan for the Arab world's most populous nation.
One man was killed by a gunshot to the neck in the city of Fayoum, south of Cairo, a local health ministry official told Reuters.
Three people were killed in clashes in the Cairo area, the security sources said. Two were shot and the circumstances of the other death were unclear.
Supporters of the Brotherhood also clashed with security forces in the city of Suez, MENA reported, as well as in Ismailia and a number of locations in the capital, security sources said.
In central Sinai, gunmen caused an explosion of a natural gas pipeline supplying an industrial zone. Nobody was hurt but the blast disrupted gas supplies to some factories in the area, security sources said.
Interim Prime Minister Hazem el-Beblawi condemned the attack on the pipeline and vowed to punish such crimes with force.
State authority collapsed in parts of the Sinai peninsula after the downfall of veteran president Hosni Mubarak in 2011, allowing hardline Islamist groups to expand into the vacuum.
Attacks on policemen and soldiers in the peninsula, which shares borders with the Palestinian Gaza Strip and Israel, intensified after the army ousted Egypt's Islamist president Mohamed Mursi last July amid mass protests against his rule.
Egyptian security forces have arrested thousands and killed hundreds of Brotherhood supporters since the overthrow of Mursi, Egypt's first democratically elected leader, and last month they declared the group a "terrorist organisation".
The Brotherhood, which says it is committed to peaceful activism, had unsuccessfully urged a boycott of the referendum on a new constitution.
State media, citing initial estimates, said around 95 percent of voters supported the new constitution, which would replace one approved under Mursi and would strengthen the state bodies that defied him: the army, the police and the judiciary.
In another sign of pressure on the Brotherhood, members of the engineers' union forced their head, identified as a Brotherhood supporter by state news portal Al-Ahram, to resign.
Unions have traditionally been seen as a gauge of Brotherhood support, in large part because the group was banned from politics during the Mubarak era. It lost its grip on another powerful professional union, representing doctors, in a vote last month. (Reporting By Shadia Nasralla; Additional reporting by Mohamed Abdellah and Ali Abdelaty; Editing by Gareth Jones)