Tension has reached a new pitch between the tiny black kingdom of Lesotho and white-ruled South Africa, which surrounds it, almost four months after South African commandos carried out a raid on members of the underground African National Congress living in the enclave.

Lesotho has accused South Africa of launching four raids on its territory last weekend and of trying to sabotage electrical installations in the capital, Maseru.

It has informed the United States, Britain, the Soviet Union and the Organization of African Unity of these complaints. Lesotho contends that the alleged raids are part of a continuing South African effort to destabilize its black-ruled neighbors.

South Africa has denied the allegations, taking "strong exception" to the protest notes and countercharging that Lesotho is trying to perpetuate a "patently transparent deception."

South Africa means Lesotho is trying to divert attention from its inability to control a local insurgency--by the Lesotho Liberation Army, which is trying to overthrow the government of Chief Leabua Jonathan.

Lesotho announced Sunday that it had captured seven black South African policemen involved in the weekend incidents. But on Monday, Lesothan Commissioner of Police Maj. Gen. Shadrack Matela revealed that this ostensible proof of South African involvement had been released from prison without his knowledge.

Western diplomats in Maseru said in telephone interviews that they were finding it difficult to sort out what has really happened, but their main concern was that there would be a further escalation in tension between the two countries.

An unexpected diplomatic development came with Lesotho making a report on the incidents to the Soviet ambassador in the nearby radical black state of Mozambique. Confirming this on the telephone, a source in Lesotho's Ministry of Foreign Affairs said "expeditious" arrangements were being made for the ambassador to fly to Lesotho.

This has given rise to speculation that Lesotho may open diplomatic relations with the Soviet Union. The two countries agreed on the step two years ago, but because of widespread opposition within the strongly Roman Catholic kingdom no formal steps were taken.

South Africa could be expected to react adversely to the opening of a Soviet Embassy effectively within its territory. It already has accused the Soviets of a "total onslaught" against the system here of white-minority rule. The South African government says the African National Congress is being used as the spearhead, with Lesotho as an important launching pad for insurgency operations.

This is why it carried out the raid on Maseru Dec. 9, although Lesotho said at the time that the 42 people killed were all either congress refugees, not activists, or innocent Lesothan citizens.

According to the Western diplomats in Maseru, who are trying to piece together what happened, there appear to have been three incidents on or near the border with South Africa on Saturday, and one on Sunday.

One was at Peka, in the northwest, where, according to a spokesman for Lesotho's Para-Military Force, a group of infiltrators tried to cross the border from South Africa. The spokesman said they were repulsed without casualties. In the second, a white man reportedly threw a hand grenade into a Catholic mission station, killing one policeman and wounding another.

On Saturday night, according to the spokesman, a group of infiltrators were captured trying to sabotage electrical installations in Maseru. It is understood the interrogation of these captured men led to a raid on a house in Maseru. Arms and some morphine tablets allegedly found there were put on display in Maseru.

The interrogation also apparently led to the arrest of seven black South African policemen from the Maseru border post who were drinking in a bar in the town. One diplomat said he believed these were the seven men released without the police commissioner's knowledge.

The Sunday attack, according to the Para-Military Force spokesman, was at the northern border town of Hendriksdrift, when again a group of infiltrators tried to cross from South Africa. Two were shot dead, the spokesman said.

While the Western diplomats admit they have little to go by, they say all the border incidents were characteristic of skirmishes that have been taking place for months with the Lesotho Liberation Army insurgents, who often cross into Lesotho from South Africa.

Lesotho repeatedly accuses South Africa of aiding them and giving them free passage. South Africa just as repeatedly denies this, counter-charging that Lesotho is aiding African National Congress insurgents--which Lesotho in turn denies.

One incident that the diplomats say looks different was a six-hour attack Saturday on a police barracks near where Lesotho borders on the tribal "homeland" of Transkei. Lesotho charged the Transkei Defense Force, commanded by an ex-Rhodesian, was involved.