The beleaguered government of President Anastasio Somoza is planning to move decisively this week to crush a general strike that has united most major sectors of the country against him.

A source close to the government said Somoza, whose family has ruled this Central American country for 45 years, has pledged for use "all legal means" to break the strike.

The organizers of the strike predicted, however, that the protest would gain strength this week. They said some businesses here in the capital that remained open last week - such as gas stations - would be closed today.

William Baez, executive director of the 700-member Nicaraguan Development Institute, estimated Saturday that 70 percent of the businesses in the capital and 95 percent of those in provincial cities were closed.

"The only reason the strike is not 100 percent," Baez said, "is that Somoza owns the rest of the businesses."

Baez, whose organization is one of the more important business groups in the country, said the Somoza family has financial interests in the national airline, the largest hotel, a sugar plantation and processing plant, a radio and a television station, an automobile distributor, a construction company, a cement plant, and a heavy equipment distributorship.

Somoza struck back at Baez' organization Saturday by stripping it of its charter, in effect making it illegal. Earlier the government took similar action against the Chamber of Commerce, which also supports the strike.

"Now practically no one is legal," said a Nicaraguan journalist. "The business organizations are illegal, all but two political parties are illegal, most of the unions are illegal and the teachers' organizations are illegal."

United against Somoza with the business community, labor unions and opposition political parties are the Catholic Church which has suggested that the president resign - and the very active organizations of university and secondary school students, who staged violent protests in several provincial towns last week.

Most militant of all are the leftist Sandinista guerrillas, who took over the main government building last month, obtaining the release of 58 political prisoners and a ransom of $500,000.

With so much of the country against him, many observers are wondering how Somoza has held out.

The most obvious answer is the National Guard, a 7,500-man force trained and supplied with U.S. aid.

"It is a personal army," said Fernando Cardenal, a Catholic priest who has been active on the leftist side of the opposition spectrum.

"The army is monolithic under a system imposed by Somoza's father. The officers are paid very low salaries but they are given full access to opportunities for corruption."

Last week, 85 officers and men were arrested on charges of trying to overthrow the government. They apparently were unhappy that Somoza had not permitted the army to forcibly evict the guerrillas who had seized the National Palace and more than 1,500 hostages."

In general, however, opposition sources say they have little hope of enlisting help from within the National Guard, which has army, police, navy and air force units.

While the National Guard and Somoza's economic power are considered the main factors keeping him in office, many Nicaraguans also blame the United States.

"The United States is guilty for all the help it has given Somoza," said a reporter for La Prensa, the outspoken opposition daily. "Carter's letter congratulating Somoza for progress in human rights caused great indignation here."

While previous U.S. administration had given Somoza full and open support, the Carter administration has tried to dissassociate itself from him. Somoza, however, has skillfully cultivated the contacts he made as a student at West Point and her allies in Congress and at the Pentagon. These supporters have pressured the administration to continue military and economic aid to Nicaragua.

Carter's letter, transmitted in July, was intended to encourage Somoza to take more steps to provide political freedom. It appears to have had the opposite effect.

United Press International reported from Managua that Ray Molina, an aide to Somoza, said that the president was "fed up" with the administration of President Carter, and that Somoza was planning "open war" on his domestic opposition.

["As they step up violence, we will be forced to match it," Molina was quoted as saying. He also charged that there was Marxists in the State Department who opposed his non-Marxist regime, and that Washington and Moscow were plotting to oust him from power.]