Egypt's military has arrested 15 people in connection with clashes in the north Sinai city of El-Arish that left six people dead.
The arrests came late on Friday after gunmen tried to storm a police station, sparking a confrontation with security forces and the army, the country's official news agency MENA reported on Saturday.
Six people, including a policeman, a military officer and two soldiers, were killed in the clashes, and 21 people were wounded.
In the fifth attack on Egypt's natural gas pipeline to Israel since the uprising that toppled President Hosni Mubarak in February, gunmen blew up a terminal in the northern Sinai Peninsula on Saturday.
No gas has been flowing through the pipeline since the last attack on the pipeline on July 12.
Officials said fighters destroyed a terminal in al-Shulaq, the last station before the line enters the sea on its way to Israel, just 16 km from Egypt's border with the Gaza Strip.
On Friday night, nearly 150 men in trucks and on motorbikes rampaged through El-Arish, firing assault rifles in the air and driving terrified residents into their homes, witnesses said.
They rode through the deserted streets of the north Sinai city waving black flags which read "There is no God but God", before attempting to storm the police station.
Earlier, the masked men used a bulldozer to damage a statue of the late President Anwar Sadat, who was assassinated by ultra-conservative Muslim activists in 1981.
'Friday of unity'
The violence came after tens of thousands of people gathered in Tahrir Square, in the capital, Cairo, to demonstrate in what had been dubbed "The Friday of Unity and the People's Will" march.
The protesters' demands include ending military trials for civilians, seeking justice for families of those killed during the revolution, raising the minimum wage, and ensuring quick trials for former government officials.
However, a coalition of Egypt's secular groups withdrew their support, saying conservative religious groups had hijacked Friday's protests.
Al Jazeera's Sherine Tadros, reporting from Cairo, said secular and liberal political parties, including the revolution youth coalition, had addressed a news conference on Friday, accusing the conservatives of taking over the protests.
"[Secular groups] said the Islamists went against the deal ... In the early hours of the morning, [they] started taking down banners and putting up [banners] with Islamic messages," she said.
"They also say the Salafists prevented the other parties from taking their positions on the stage and essentially pushed them out."
The assistant head of the Wafd Party, the oldest and largest secular party, urged the Muslim Brotherhood to come out and declare that it had no intentions of forming an Islamic state.
Several banners reading "Islamic law above the constitution" were displayed in Tahrir Square and protesters who fear Islamists will seek to dominate plans to rewrite the constitution demanded the banners be taken down. Similar tensions emerged in Suez.
However, Essam al-Erian, vice-chairman of the Muslim Brotherhood's Freedom and Justice Party, said on the party's website that he rejected the actions of Islamists trying to impose their vision on people in Tahrir.
He criticised the provocative slogans used by some Islamists and said the party rejected these "minority demands".
Friday's rally came a day after the country's justice ministry announced that the trial of former President Hosni Mubarak and others accused of involvement in the killing of protesters during the country's uprising - including the former interior minister and six senior police aides - would be held on August 3.
"It has been decided that the trial of ex-president Hosni Mubarak and his two sons Alaa and Gamal ... will be held in the building of the General Authority for Investment and the free trade areas in the Cairo Expo grounds," the official news agency MENA quoted a justice ministry official as saying on Thursday.
Mubarak has been in hospital in the Red Sea resort of Sharm el-Sheikh since April, when he was first questioned by the authorities.
He has been charged with involvement in the killing of protesters and abuse of power but it was not immediately clear if Mubarak's health condition would prevent him from standing trial in person.
Amr Hilmy, the health minister, told reporters that "Mubarak's health is in an appropriate condition to be tried in Cairo".