Guatemala's capital is filled with rumors of imminent war - possibly as early as the weekend - with Britain over the future of neighboring Belize.

The Guatemalan government has threatened to invade Belize if Britain grants it independence unilaterally. Last week, on the eve of renewed talks with Britain which opened today, Guatamala took a tougher line.

Now, it says there must be substantial movement toward a settlement during this round of talks, or President Kjell Laugerud may feel compelled to take other measures.

Guatemalan Foreign Minister Adolfo Molina Orantes and British Secretary of State Ted Rowlands led large delegations to the two-day talks at the headquarters of the Organization of American States. The Belizean prime minister and opposition leader were in the British team.

Neither side would comment on the discussions but a British spokesman said, "we are not complaisant" about the rumors of confrontation. Although the atmosphere reflected tension, both sides were holding lunches for the other delegation.

The British spokesman denied that reinforcements had been sent for what he said were 1,300 troops in Belize. In London, a Foreign Office spokesman said that the dispute boils down to what sort of aid Guatemala requires to drop its claim.

Guatemalans are very nervous about the possibility conflict and not confident of victory. Although Belize would not be able to defend itself against Guatemala's well-equipped, U.S.-assisted army, published reports here have said Britain has plans to rush in sufficient reinforcements in 48 hours to repulse any invasion.

Guatemala has about 10,000 troops and accounts here put the size of the British garrison in Belize at 2,500.

Guatemala reported on Monday that a British warship had entered Belizean waters and the army denounced the ship's arrival, saying that it "constitutes a manifest demonstration of aggression and an absolute violation of international law."

Two weeks ago, the government began an intensive propaganda campaign to prepare the citizenry for drastic measures. Messages castigating the British "usurpers" and listing their alleged affronts to national dignity are broadcast every half hour on television.

On June 3, Army Day, only students and reservists marched.Usually, the cream of the army parades through the center of the city and their absence only seemed to confirm the rumors that the regular troops were being prepared to invade Belize.

The Governor has issued only generalized warnings and says that it still hopes for a peaceful solution.

Numerous low-level officials are advising their families and friends to stockpile food however. One official said the army was conducting sweeps to induct able-bodied men 18 to 36.

The British consul here is brusque in denying the possibility of war.

Some informed sources say Guatemala may be trying to pressure Britain, and even more, the United States, hoping that it will step in and resolve the dispute. "They know that what the U.S. is most concerned about is hostilities," one source said.

There have been no recent troop movements toward the frontier, sources said. The Guatemalans have kept large concentrations near the border since 1975, however.

President Laugerud recently accused Britain of acting in concert with Cuba, Panama and Belize to forment terrorism within Guatemala.

Aside from upholding the traditional claim "Belize is ours," taught to all Guatemalan schoolchildren, the rightist military men who rule Guatemala fear that an independent Belize will become a conduit of Caribbean radicalism and a haven for leftist guerrillas.

Belizean Prime Minister George Price has asked London for a post-independence defense guarantee, but so far the British have refused.

Economically, Belize could be self-sustaining. It has balanced its budget without external aid since 1967, and large oil deposits are believed to lie off its coast. American companies are to begin drilling there soon.

The Guatemalan army believes that Cuban troops are in Belize. British and U.S. intelligence sources deny it. Price had said, however, that if Guatemala does not relent in its claims, he would have to seek some means to secure Belize's independence.

The racial differences form a large unspoken part of the uneasiness that exists on both sides. Most Belizeans are black, descendents of slaves the British brought in to work the logging trade. Belize sees its natural allies in the block of English-speaking Caribbean island states, while the Guatemalans view the territory as a wedge in the solid line of Central American conservative governments.

Though Guatemalan claims go back to colonial times, they base their main argument on the 1859 treaty that defined present-day borders. In exchange for large territorial concessions, the British promised to help build a "cart road" from Guatemala City through Belize to the Carribean. Since the road was never constructed, Guatemala contends that the colony, or at least the 1859 concessions, should revert back to it. They point out that this treaty represents the only legal title Britain has for its continued presence.

The British consul in Guatemala, William MacQuillian, said the claims are "without substance". The crucial point, he stated, was that neither colonial Spain nor independent Guatemala ever controlled the area. The 17th century English pirates who first sailed up its rivers seeking refuge found only uninhabited marshland. The British are willing to negotiate, but MacQuillian stressed that its policy is to arrange "complete independence as soon as possible."

The Guatemalans also say that they want to solve the problem peacefully, but they have worked themselves into a difficult corner. Their 1944 constitution incorporates Belize into the nation, so that any president would be violating if he agreed to permit Belize's independence.

The issue is a fierce nationalistic cause here, constantly in headlines. Bumper stickers and radio announcements proclaim Belize "an integral part of the fatherland."

Three seats in the Guatemalan Congress remain vacant, reserved for Belize's "liberation."

In 1954, Guatemala broke off diplomatic relations with Great Britian when it granted internal self-government to the colony, and in 1975 Britain had to reinforce its garrison in response to a Guatemalan troop buildup across the border.

The controversy was taken to the U.N. General Assembly in 1975 and 1976. Both times it voted overwhelmingly in favor of independence for Belize. The United States abstained.

Guatemala broke off diplomatic relations with Panama in May, because Panamanian leader Gen. Omar Torrijos broke ranks with the traditional Central American bloc position of favoring Guatemala's claim and said he favored independence for Belize.

The United States says it is trying to stay out of the dispute. On May 20, the State Department declared that "the question of Belizean independence is a matter to be worked out among the parties directly concerned: the British, the Guatemalans, and the Belizeans."

The United States, however, remains the preponderant power in the area. Both sides have turned to Washington for mediation in the past, and if trouble does break out, they will probably do so again.

Hugh O'Shaughnessy of the London Observer reported from London:

Israel is selling arms to the hawkish generals of Guatemala.

Twenty-six tons of Israeli arms en route to Guatemala were seized from an Argentine plane in Barbados June 25. Barbados protested to Israel because it opposes Guatemala's claim to Belize and fears the arms shipment is related.

[A British embassy spokesman in Washington said that Israel has sold Guatemala about 10 shot-takeoff-and-landing fighters - "ideal for the terrain in Belize" - as well as rifles, in addition to the arms seized in Barbados.]

Israel has been supplying aircraft to Guatemala for a number of years, having first provided Arava troop transports that could drop paratroops in Belize.