A purge of one faction of the beleaguered Moscow-installed Afghan government has "crippled" its activities and left Soviet advisers making all major decisions, diplomatic sources from the Afghan capital of Kabul reported here today.

As a result of the political tensions caused by the purge, which started late last week, authorities have tightened security in Kabul and ordered an unusual closing of schools for 20 days, according to the diplomatic reports and usually well-informed Afghan refugee sources here.

A Western diplomat in Kabul informed his government that there has been an increased presence of Afghan military forces in Kabul since Saturday, when tanks took up posts at every bridge crossing the Kabul River and at other strategic points in the city.

Self-propelled artillery pieces were stationed just outside the city and more armed soldiers and civilians were seen patrolling the streets, the diplomat reported.

The purge reportedly is aimed at ridding the Afghan government of supporters of two slain presidents, Nur Mohammed Taraki and Hafizullah Amin, who were leaders of the Khalq wing of the Marxist People's Democratic Party of Afghanistan.

The sources said the moves were also intended to increase the personal power of President's Babrak Karmal, the head of the party's ruling Parcham faction, whom the Soviets installed last December when they invaded Afghanistan and overthrew Amin.

The purge is seen be diplomatic observers as a setback for the Soviets, who tried to forge the two party factions into a common front that could run the country under Babrak.

Instead of getting together, members of the two factions have carried out a feud with nightly assassinations that brought government activities to a standstill.

"The Khalqs were not prepared to forego the taste of power without bringing about an atomsphere of tension and turmoil which they though might frighten the Russians into not accepting all the terms of the Parchams," said one former Afghan government civil servant now in exile in New Delhi.

Beyond that, there have been intense personal hatreds betwen high government officials who belonged to the rival factions. Members of the Khalq faction, for exampled, are known to have jailed, tortured and killed Parcham members and their families when Taraki and Amin were in power. Babrak was forced into exile after Taraki took over the government in a bloody coup in April 1978.

According to diplomatic observers here, Babrak is on shaky ground in pushing the purge since it will further decimate the already thin cadre of experienced government officials.

Moreover, Khalq make up two thirds of the 75,000 members of the ruling party and fully 80 percent of the remaining officers of the Army and Air Force, which are already riddled with desertions by both officers and enlisted men.

In one of the first moves in the purge, Babrak Sunday all but wiped out the Interior Ministry. As part of an "administrative reform," he shifted its powers to the office of the prime minister, a title he holds in addition to those of president and secretary general of the ruling party.

Babrak started the purge a week ago when he placed Parcham supporters in key provincial posts under the Ministry of Finance. A former Afghan official now living here said these posts had been the backbone of support for the Khalqs.