Rhodesia today completed the last phase of a general military mobilization for next week's elections of the country's first black-led government and put its vastly expanded security forces on high alert.

At the same time, it kept up its pounding of nationalist guerrillas opposed to the elections and based in neighboring Zambia. Carying out its fourth raid in two days, the Rhodesian Air Force hit a camp in far northern Zambia near the Zairian border, killing 136 persons and wounding 200 others.

In another development today, the government announced it had arrested "a limited number" of persons who it said were known to be supporting the guerrilla attempt to disrupt the elections. But it gave no precise figure.

With martial law now covering 90 percent of the country, no one outside the inner circle of government seems to have any idea how many Africans are being held as political detainees. The number is thought to run into the many hundreds.

Hundreds of white and a few non-white reservists reported for duty this morning at police and army barracks here in the capital and were immediately sent off by truck to various stagging points throughout the country.

The government has now mobilized between 60,000 and 70,000 police and army troops to deal with the guerrilla threat to disrupt the elections, roughly doubling the normal size of its security forces.

Almost all able-bodied white men, including 7,000 between the ages of 50 and 59, have been called up for the occasion, paralyzing many companies in the process.

"You've got a job and you have to do it," said mixed-race restaurant owner Anthony David, 54, going through a crash five-day course in handling arms for 110 members of the so-called Grandpa's Army. "The election must work. People are tired of the war and killing."

David, who had not seen military duty since 1952, left his wife to take care of their restaurant, the Himalaya, while on duty, so his business will not suffer greatly. But many other small businesses will and some are simply closing altogether.

One large Salisbury firm of accountants employing 2,200 whites and blacks has given everyone a three-week vacation starting with Good Friday. Most larger companies, however, are limping along on skeleton staffs, including the country's main newspaper, The Herald.

The police and army reservists now being mobilized for the elections are being used mostly to guard fixed positions, like bridges and vital installations, to man roadblocks around the towns and cities and provide protection for the 700 polling booths, 240 of which are mobile.

This will allow the small regular Army to make full use of all its troops to deal with the expected offensive by the estimated 12,000 guerrillas now inside Rhodesia.

Rhodesia's standing Army numbers only about 9,500, but with a continuing callup of reservists it is much larger at all times now.

With the police, the security forces normally number 30,000, giving them a slim 3-to-1 edge on the guerillas.

This helps explain why the government counts heavily on air and grounds attacks into Mozambique, Zambia and Angola to deal with the mounting guerrilla threat.

So far this year, Rhodesian jets and bombers have struck six times at guerrilla camps in Zambia, six more in Mozambique and once in far-off eastern Angola.

Today's raid was against a guerrilla camp near a refugee center 60 miles southwest of Solwezi in northern Zambia, a military communique said.

Many guerrillas and refugees, who number more than 50,000 now in Zambia, were recently moved from sites around Lusaka to the far north to give them greater protection from Rhodesian raids.

The camp at Shelinde is more than 300 miles from the nearest Rhodesian air base at Kariba.

Zambian President Kenneth Kaunda said in Lusaka today that in the past two days 138 nationalist guerillas or refugees had been killed and 219 others injured.