Opposition Labor Party leader Neil Kinnock, struggling to regain authority after two serious setbacks at the party's convention here, today delivered a powerful warning to his quarreling membership to remember the taste of defeat last year when "we were stunned by our worst postwar election" loss.

Kinnock combined a scathing attack on Conservative Prime Minister Margaret Thatcher with what his aides hope will be reassurance to the British public that Labor is committed to democratic and legal methods despite the turmoil and violence of a coal miners' strike that has produced great strains and divisions within the traditional party of the working class.

Kinnock sought to ease harsh feelings between the party and British police, which were intensified yesterday when more militant party leaders passed a resolution condemning the police alone, and not striking miners, for "unlawful" picket-line violence. Passage followed a rousing speech by mine union leader Arthur Scargill.

And at a time when court orders are being flouted, some by Scargill who mocked a contempt citation against him yesterday, Kinnock also sought to move the party away from notions that some laws can be violated.

"We cannot sharpen legality as our main weapon for the future and then simultaneously scorn legality because it doesn't suit us at the present time," he said.

The 42-year-old Welshman, who was elected one year ago amid calls for party unity after a Conservative landslide, is a dramatic and articulate populist-style speaker, and his speech here was warmly applauded. But Kinnock is relatively new and untested in national politics, and he has suffered a series of sharp blows in recent days that will make the conference and the reaction to his performance today particularly important.

After steady gains in the opinion polls that had brought Labor alongside the Conservatives, two polls published this week show sharp declines in popularity for both Kinnock and Labor.

Yesterday, a majority of the 1,300 delegates rejected Kinnock's efforts to change the system by which Labor members of Parliament are renominated. The change would have taken power away from small groups of activists in favor of a vote open to all members.

The existing system potentially threatens several of the senior but more moderate figures in the party who are members of Kinnock's shadow cabinet, and Kinnock staked his reputation on changing it.

The defeat has shaken party officials, because Kinnock seems to have badly miscalculated his support and wound up losing his first major test of confidence on an issue that many members felt was not worth the cost.

Similarly, the resolution against the police appalled numerous moderates in the left-of-center party because of its one-sidedness in the face of strong public expressions of support for the police and belief that it is the picketing miners who are causing the violence.

"It was a political error for us," said Labor member of Parliament John Cunningham.

Last month at the huge Trade Union Congress, Kinnock condemned violence on all sides but focused heavily on the miners as causing the violence.

Today, his criticism was more even, but he said the police were in a tough position.

"I condemn the violence of the stone throwers and the battering-ram carriers," he said of the miners, "and the cavalry charges and truncheons" of the police. But the police, he said, were being used by Thatcher "as a replacement for policy" and as "the meat in the sandwich."

Kinnock and many others said that the violence is a reflection of despair and frustration resulting from record unemployment and unfair tax systems. "It is the product of Thatcherism -- the combination of ignorance and arrogance that now rules this country and makes Britain less free, less fair, less productive with every passing year," he claimed.

Today, Kinnock's dual condemnation of pickets and police was applauded. Yesterday, two union leaders who got up to challenge Scargill were shouted down.

Erice Hammond, general secretary of the electrical workers union, warned that it was "shameful" not to condemn the miners' picket-line violence and warned that this would haunt the party for years, a view shared by many of the moderates in the party's leadership.

He also perservered through the catcalls to say that there had been no party attempt to analyze the real problems in the coal dispute, to challenge the refusal of the miners to have a national ballot of their members or to abide by trade union rules on disputes.

Although Kinnock is well to the left in his party, he has sought to distance Labor from some of the more extreme views that opinion polls show were a major reason for their last two drubbings in national elections. And he also sought today to end the battle within the party between extremists and moderates, which has frightened voters.

For a political party, he said, "amnesia can be a crippling disease. In the Labor Party, the vaccine is to simply recall what defeat felt like last year and that should immunize every one of us completely against any attitude or activity that impedes our progress toward victory."