Guatemala's military government has arrested without charges eight civilian leaders from the country's three main right-wing political parties, in what appeared to be a continuing clampdown on political activity since the declaration of a state of siege six weeks ago.

The government announced yesterday that security forces were hunting for an undetermined number of other civilian leaders still at large, including Leonel Sisniega, a top leader of the National Liberation Movement and an early supporter of the March 23 coup that brought President Efrain Rios Montt to power, according to a Guatemalan journalist reached by telephone in Guatemala City.

State Department spokesman Alan Romberg, when asked about the arrests at a briefing yesterday, said "incidents such as this indicate the difficulties the government is having in bringing about change in Guatemala. We continue to support the Guatemalan government's efforts to end corruption and human-rights abuse . . . ."

The Reagan administration has said it intends to seek congressional approval for military aid in the Guatemalan Army's battle against a guerrilla insurgency. Last week, Deputy Secretary of State for Latin American Affairs Stephen Bosworth told a congressional subcommittee that since Rios Montt took power, the government's record "while not perfect, demonstrates that the government has a commitment to positive change."

The government announced the arrests Saturday, saying only that the politicians had violated "laws in force" -- an apparent reference to a ban on political party activity under the July 1 state of siege, which also gives the government broad power to hold people without charges.

In Guatemala City, the local press linked the arrests to a speech two weeks ago by Rios Montt, in which he said the government had discovered plans to overthrow his government this month.

Reports from the Guatemalan capital said the men under arrest were flown Sunday to the remote northern province of Peten, which borders on Mexico, and are being held at the Army base of Flores, the provincial capital.

Those in custody reportedly include three leaders of Sisniega's party, a powerful group that has its own armed paramilitary force of several thousand men and is considered to be well connected in the military.

The other five under arrest are from the party of former president Carlos Arana Osorio, the National Authentic Central, and from the Popular Democratic Front, led by former defense minister Angel Anibal Guevara. Guevara, the official government candidate, won the March presidential elections, which were widely considered to be fraudulent and were set aside by the coup.

Sisniega was the National Liberation Movement's vice presidential candidate in the elections. The top party leader, Mario Sandoval, was at his home in Guatemala City yesterday. He said through an intermediary that "nothing is clear" about the arrests, but that he "had not been accused of anything."

All those arrested were "middle-level" leaders in their parties, the Guatemalan journalist said. He said the arrests had caused "a lot of speculation," because of the rightist political affiliations and closeness to the military of all those arrested, adding that the government action would make the country's return to democracy "difficult."

Rios Montt at first ruled as part of a three-man junta of Army officers, promising to improve the country's human rights reputation. In June he consolidated his power, ousting the other junta members.