LONDON, NOV. 28 -- John Major took office as Britain's prime minister today, seeking immediately to bind up his party's wounds and assert his independence from predecessor Margaret Thatcher by naming his main campaign rival to a politically sensitive cabinet post.

Major, who was Thatcher's protege and choice to succeed her, appointed Michael Heseltine as secretary of state for environment and local government. The job gives his former opponent -- a bitter rival of Thatcher who helped topple her from the premiership -- responsibility for revamping or scrapping altogether one of the most unpopular measures of Thatcher's years, the new system of local taxation known as the poll tax.

Heseltine's appointment was part of a broader shake-up in which eight cabinet positions changed hands, a higher number than most analysts had expected. The newcomers are generally younger than their predecessors and are of varied shades of ideology within the Conservative Party, in keeping with Major's pledge to unify the party after the sudden and wrenching leadership change.

The new cabinet is all-white and all-male. In a brief speech in front of 10 Downing Street after assuming office, Major praised "the enormous achievements that I inherited from Margaret Thatcher" and said he was certain "history will record that she was a towering prime minister."

But Major signaled a change from the divisive, confrontational style of politics that Thatcher practiced during her 11 years in power, saying he wanted to "build a country that is at ease with itself, a country that is confident and a country that is prepared and willing to make the changes necessary to provide a better quality of life for all its citizens.

"I don't promise you it will be easy and I don't promise you it will be quick. But I believe it is an immensely worthwhile job to do."

Major, 47, a high school dropout who soared through the cabinet to Britain's highest office, also signaled a more conciliatory approach to European economic and political integration. He said Britain had before it "a decade of the most remarkable opportunities. . . . We have in front of us the building and development of an entirely new Europe . . . in which this country will play a full and leading role."

Heseltine, 57, is a former defense secretary who stormed out of Thatcher's cabinet in protest nearly five years ago and made his opposition to the poll tax the cornerstone of his campaign against her. Many in the party will be dismayed to see him return to government in a role in which he could dismantle one of her pet projects, but many also say drastic changes in the tax are necessary for the party to win the next general election.

In the past, Heseltine has advocated removing the cost of paying state school teachers from local governments and pegging the poll tax to income levels to make it less regressive. But some Tories say the only real solution is to scrap the measure and return to the old system of property taxes.

The prime minister named Norman Lamont, his campaign manager and close ally, to Major's old post of chancellor of the exchequer. Lamont, 48, was Major's deputy at the treasury, where he had a reputation as a budget chopper and ardent free marketeer.

Lamont is also a self-declared "Euro-skeptic" and is expected to take a tough stance against further surrender of sovereignty when he attends the European Community's summit conference in two weeks.

Another high-profile cabinet member is Chris Patten, 46, who is moving from environment secretary to become chairman of the Conservative Party. In that role, Patten, an articulate, well-respected member of the party's liberal wing, will serve as campaign manager and main spokesman for the Conservatives as they gear up for the next election, which must be held by the summer of 1992.

David Mellor, 41, a close friend of Major on the party's left, takes Lamont's old job as Treasury chief secretary. Ian Lang, 50, becomes Scottish secretary. Major shifted home affairs secretary David Waddington to leader of the House of Lords, gave Waddington's old post to party chairman Kenneth Baker and moved Scottish secretary Malcolm Rifkind to transport secretary.

The best-known casualty of the reshuffle was Cecil Parkinson, a former party chairman who was one of Thatcher's closest allies and a possible successor until a personal scandal ended his rise. He resigned this morning rather than face the prospect of being fired.

Major kept in place Foreign Secretary Douglas Hurd and Defense Secretary Tom King, sending a clear message that he intends to maintain Britain's support for the United States in the Persian Gulf crisis. He also kept on Health Secretary William Waldegrave and Education Secretary Kenneth Clarke, two young and liberal ministers, despite the fact both had campaigned for Hurd in the three-way contest to succeed Thatcher.

"There's no ill feeling at the end of this contest," said Major in his speech. "At the end of this week I believe there is a smile on the face of the party. That will mean we are fully united for the future."

Political opponents said the new cabinet was a mere imitation of Thatcher's last group. "The key jobs are still in the hands of Thatcherites," Jack Cunningham, campaign director for the opposition Labor Party, said.

Most Conservatives welcomed the changes, although some grumbled over Heseltine's appointment and over the lack of a woman member. "I didn't vote for John Major to end up with a 100 percent loss of women in the cabinet," said Teresa Gorman, one of several women lawmakers who had hoped for a position.

"We need somebody in there to put {across} a woman's point of view."

Major's accession to the premiership marked the formal end of a tumult that began eight days ago when Heseltine won enough votes from Conservative members of Parliament to block Thatcher from winning in the first round of leadership balloting and to force her out of the race.

Thatcher emerged from Downing Street for the last time as prime minister at 9 a.m. today and fought back tears as she briefly recalled her 11 "wonderfully happy years" in office.

"Now it's time for a new chapter to open, and I wish John Major all the luck in the world," she said.

Thatcher then rode to Buckingham Palace, where she formally handed in her resignation. After a half-hour session with Queen Elizabeth, Thatcher drove off with husband Denis and son Mark to her new home in Dulwich, southeast London.

Minutes after Thatcher left the palace, Major and his wife Norma arrived for the formal, private ceremony at which the queen handed him the seals of office.