About 1,000 protesters were standing in the road to block the entrance to the industrial area of the coastal town of Sohar, which includes a port, refinery and aluminium factory.
Hundreds more were protesting at a main roundabout, angry after police opened fire on Sunday at stone-throwing protesters demanding political reforms, jobs and better pay. Protesters later burned the town's police station and two state offices.
"We have received a total of six deaths yesterday from the protests in Sohar," an emergency doctor at the state hospital in Sohar said.
Witnesses had earlier put the death toll at two. Several said police had used rubber bullets but at least one witness said they fired live ammunition.
The unrest in the northern port of Sohar, Oman's main industrial centre, was a rare outbreak of discontent in the normally sleepy sultanate and followed a wave of pro-democracy protests across the Arab world.
In Sohar, a main supermarket was burning on Monday morning after being looted, witnesses said. Troops deployed around the town but were not intervening to disperse protesters.
Exports of refined oil products from Sohar's port were continuing although the flow of trucks into the port was blocked, a port spokeswoman said.
Sultan Qaboos bin Said, trying to ease tensions in U.S. ally Oman, reshuffled his cabinet on Saturday, a week after a small protest in the capital Muscat. He has ruled for four decades, exercising absolute power. Political parties are banned.
The government, under pressure over its response to the Sohar protests, pledged on Sunday to create 50,000 more government jobs and hand out unemployment benefits of $390 a month to job seekers.
Mostly wealthy Gulf Arab countries have stepped up reform measures to appease their populations following popular unrest that toppled the leaders of Tunisia and Egypt and is threatening the position of Libyan leader Muammar Gaddafi.
Oman is a non-OPEC oil exporter which pumps around 850,000 barrels of oil per day, and has strong military and political ties to Washington. Sultan Qaboos deposed his father in a 1970 palace coup to end the country's isolation and use its oil revenue for modernisation.
He appoints the cabinet and in 1992 introduced an elected advisory Shura Council with 84 members. Protesters have demanded the body be given legislative powers.
On Sunday, Qaboos ordered a ministerial committee to study increasing its authority.