Soldiers in trucks and armored cars took to the streets with orders to halt violence between law enforcers and opposition party supporters that has left at least 20 people dead in three days.
Prime Minister P.J. Patterson called out the army on Monday night.
"The government cannot stand idly by and allow criminal elements to hold this country to ransom," he told reporters.
"There must be full deployment ... to prevent anticipated problems and to ensure the restoration of law and order."
Step One: Kingston
It was not clear how many soldiers were involved. They mainly moved into the streets in the capital, Kingston, as helicopters hovered overhead. Patterson said they also would deploy in other hotspots around the Caribbean country. The military was given police powers, mostly to maintain cordons and enforce curfews, he added.
Leaders of Jamaica's two main political parties — Patterson and Labor Party leader Edward Seaga — each accused the other of inciting the violence ahead of general elections scheduled to take place before the end of next year.
The violence grew out of more than two months of clashes between gangs with rival political ties. Thirty-seven people were killed in those fights.
But at least another 20 have been killed since Saturday, when police and soldiers moved into Kingston's notoriously dangerous Tivoli Gardens neighborhood, a Labor stronghold.
"The police are downtown killing people, but only Laborites," said 48-year-old Beverly Brown, among 30 protesters at a makeshift roadblock in the Grant's Pen neighborhood.
Police said snipers hidden in buildings strafed the streets with automatic gunfire, and some people attacked them with homemade bombs.
Demonstrators mounted roadblocks, including in the northern tourist resort of Ocho Rios. Many said they were protesting a police crackdown on opposition neighborhoods.
Those killed in the latest fighting included two police officers and a soldier, but police have not identified any of those killed since Saturday as gang members.
Even Before the Violence, a High Murder Rate
Jamaica, with a population of 2.6 million, has one of the highest murder rates in the world. About 530 people have been murdered this year.
That figure does not include about 75 people killed by police between January and last week — killings the police say are in self-defense and that human rights groups charge are summary executions.
Patterson mounted a similar draconian crackdown two years ago, calling out the army and putting troubled neighborhoods under curfew. The government said that it brought the murder rate down by 11 percent in 1999 and that other crimes fell, including robbery and rape.
Jamaica's political history is intertwined with the gangs the two main parties helped organize and arm in Kingston's poor neighborhoods in the 1970s, gangs that controlled the streets at the behest of politicians and intimidated voters at election time.
The political violence reached its height in 1980, when an estimated 700 people were killed in election-related fighting.
Then the gangs turned to lucrative cocaine and marijuana trade, making them financially independent, though most retain political allegiances.
On Monday, the fighting and demonstrations shut down banks, stores and other business throughout Kingston. Demonstrators also looted three stores in Spanish Town, near Kingston, police said.
The disturbances could hurt Jamaica's vital $1.3 billion tourism industry, though the violence is concentrated in Kingston, not tourist resorts in the north and west.