Amnesty International today charged Pakistan's martial- law government with increased use of torture and mass arrests over the past year to throttle political opponents in a way that has resulted in a "steady deterioration in respect for human rights."

The Nobel Prize-winning human rights organization said in a report issued in London that the military government of President Mohammed Zia ul-Haq had jailed 6,000 political prisoners last March--a figure disputed as far too high by both the Pakistan government and many independent observers here.

Nonetheless, the 54-page Amnesty report detailed how new regulations issued during the year have crippled the ability of the legal system, especially the previously independent judiciary, to protect individuals from government repression.

The most serious Amnesty charge was that the systematic use of torture against political prisoners has increased over the past year and that at least 10 persons have died as a result of torture or ill-treatment in jail between January 1980 and August 1981.

Some of the allegations of torture were backed by medical statements that describe scars "so strongly suggestive of being caused by torture that they should be accepted as confirmatory evidence," the report said.

Besides denying the specifics of the report, including the number of political prisoners arrested and the use of torture, the Zia government appeared this week to be taking steps to change its image of being a harsh, authoritarian regime.

These included an end to 27 months of prior censorship of newspapers and its replacement by editorial self-censorship that could, in fact, be more restrictive, and the convening of an appointed national advisory council that is seen as a tentative step toward ending martial law.

Even with these apparent moves toward the liberalization of the 4 1/2-year-old military government here, however, the Amnesty report contains potential embarrassments for Zia.

Overall, it further darkens the government's image at a time when Zia would like to improve it and it could weaken the impact of his forthcoming trip to Europe--including state visits in Romania, Italy and possibly France--by handing new ammunition to his overseas critics.

The report is, moreover, likely to bolster congressional opponents of Reagan adminstration plans to make Pakistan a front-line state against Soviet expansion into the Persian Gulf with a five-year, $3.2 billion military sales and economic aid package.

Regionally, the report could strengthen India's hand in negotiations over a no-war pact between the two nations by enhancing New Delhi's claim to being a democracy that must remain vigilant against an authoritarian military government on its border.

Although Amnesty International has been critical of other Pakistani governments in previous reports, there was no attempt in today's study to compare the Zia administration's record on human rights to previous governments, including that of Zulfiqar Ali Bhutto who was executed by Zia after being found guilty of political murders.

Bhutto, the elected prime minister whom Zia overthrew in July 1977, is cited by many Pakistanis not sympathetic with the present regime as having been far more authoritarian in his last years than the current military rulers.

Zia, for instance, released 6,000 political prisoners held by Bhutto when he took over.

Nonetheless, as the Amnesty report pointed out, Zia ordered far-reaching changes in the legal system that make it almost impossible for a political prisoner to have the kind of open trial, with the right of appeal, that Bhutto had before he was hanged in the Rawalpindi jail courtyard in April 1979.

These changes, in an order issued last March, subjugate the civilian judiciary to the military courts and, in effect, end many constitutional protections to individual liberties.

"It marks the virtual end of the independence of Pakistan's judiciary," Amnesty's report said.

While acknowledging that the Zia government's practice of arresting, releasing and then rearresting political opponents makes it difficult to get an exact count of the number being held at any one time, Amnesty charged that 6,000 persons were picked up last March for political activities.

The government, although refusing to comment on the report, strongly disputed that figure and, for the first time, gave its version of the number arrested, saying it was 300.

While there is no question that a series of antigovernment events--including the hijacking of a Pakistan International Airlines plane to Kabul and Damascus by an anti-Zia terrorist group headed by Bhutto's son Murtaza--sparked mass arrests in March, the Amnesty figure is considered much too high by most independent observers here.

According to many diplomats, including some from countries not generally considered friendly to Pakistan, the number of those detained was no higher than 2,000. [An Amnesty International spokesman in New York said Tuesday that he had no comment on the estimates of the number arrested in March made by the observers in Pakistan.]

At present, a government spokesman said, there are 16 well-known political figures and 62 minor political workers under detention.

The State Department in November said there were then fewer than 500 persons in jail for political reasons. [The Amnesty spokesman also said he had no comment on the State Department figure on the number detained in November.]

The best known of these are Benazir Bhutto, the firebrand 27-year-old daughter of the executed former prime minister and leader with her mother of the Pakistan People's Party, and retired Air Marshal Asghar Khan, an outspoken opponent of Zia who takes to the stump against the military government any time he is freed. Both are under house arrest.

For the first time, Amnesty said, there are credible stories that women are being tortured. The names of three women who said they suffered ill-treatment at the hands of Army or police questioners were given in the report.