President Mohammed Zia ul-Haq today ended military rule in Pakistan, 8 1/2 years after he imposed it upon taking power in a coup d'etat.

Declaring, "I hand over the reins of the country to the people," Zia signed a decree transferring authority from the armed forces to a civilian government of his own design, over which he will continue to wield decisive political power.

While Pakistani and foreign observers say they do not believe the carefully prepared handover will markedly change the lives of Pakistan's 90 million people, the return to civilian rule has been described as essential to maintaining the country's stability at a time of growing economic and political pressures. In a televised speech to Parliament, Zia also announced the restoration of several articles of the constitution, including those guaranteeing basic freedoms.

Most observers -- including diplomats and Pakistani politicians and intellectuals -- supported Zia's move as a step toward democracy. But many suggested that the manner of his handover had left the country in a state of constitutional confusion, in which various laws -- and some martial-law orders, which remain in force -- will continue to give Zia the power to manage Pakistani political life.

Zia remained vague on whether he will give up the post of Army chief, which provides tight control over the military structure. Under Army rules, he must step down with the ending of martial law. In an interview in October, Zia said he would end his military role as leader of the armed forces when martial law was lifted.

As civilian president, Zia will keep broad powers, including a veto over all legislation, the authority to dissolve Parliament and, under pending legislation, control over a commission empowered to license and disqualify political parties, which were banned until now. But theoretically he will give up day-to-day control over the government.

The United States, which relies on Pakistan as an important element of its strategic concerns in southern Asia, poured $3.2 billion in aid into the country over the past five years, an amount Zia would now like the Reagan administration to double.

[In Washington, State Department spokesman Charles Redman said, "We heartily welcome this decision by the government of Pakistan which, by this step, has fulfilled its pledge to restore full constitutional government."]

Opposition political leaders of the 11-party Movement for the Restoration of Democracy, who have opposed Zia's careful control of the transition to civilian rule, continued to denounce it as a sham. In particular they condemned his recent amendments to the constitution and his plan to remain as Pakistan's president until the spring of 1990 on the strength of a referendum a year ago that many, even diplomats generally sympathetic to Zia, charged was rigged.

Earlier this month, the Karachi Daily Dawn remarked that by keeping the martial-law decrees, "we will be governed by hundreds of supraconstitutional instruments in conflict with one another and with basic law."

Most observers agree that the change of administration will have little effect on circumscribed political liberties in Pakistan.

One immediate effect was the closing of martial-law courts, in which military officers -- many without legal training -- handed down stiff penalties, including lashings, against which there was no appeal. The military courts had been widely criticized by opposition parties and the human rights organization Amnesty International for their handling of political prisoners.

Although the lifting of martial law removes a blanket ban on political party activities, a pending new law, which Zia demanded as a condition for civilian rule, will require parties to be licensed. The law gives the country's election commission -- whose chairman was appointed by Zia -- broad latitude to disqualify parties judged to deviate from Islamic ideology or to be too critical of the armed forces or the judiciary.

Western and Pakistani observers say such rules are aimed at opposition groups such as the Pakistan People's Party, which espouses an "Islamic socialism."

Such restrictions accord with Zia's personal opposition to party politics in Pakistan. Today, he blamed parties for much of the past turbulence and violence of Pakistani politics, and called on Parliament to consider a partyless political system.

Nonetheless Prime Minister Mohammed Khan Junejo announced yesterday he would revive the Moslem League, the party founded by Pakistan's first leader, Mohammed Ali Jinnah.

For many, the biggest surprise of Zia's speech was his decision not to declare a state of emergency under his new civilian government. In a statement thanking Zia, Junejo noted that it marked the first time in 20 years that Pakistan has not been governed under emergency powers.

Zia took power in July 1977, toppling president Zulfiqar Ali Bhutto, who was later hanged. At the time of the coup and again two years later, Zia promised national elections only to cancel them. That history has fueled skepticism among Pakistanis of his declared intention to move the country toward a broad-based democracy.

In his speech, Zia defended at length his 1977 takeover, saying that political upheaval over disputed elections had threatened the country, and describing his martial-law administration since then as a necessary "life-saving drug."

Observers say the most important question is how tight a leash Zia will keep on his Parliament and prime minister.

"Junejo is not an aggressive politician," a Pakistani newspaper editor said last fall. "Both he and the Parliament members have achieved a certain measure of power and prestige, and most are not interested in asserting themselves against Zia's wishes."

Concern over the Army's response to civilian rule seering with some of his work.