TORONTO, JUNE 7 -- Prime Minister Brian Mulroney and Canada's 10 provincial premiers today were reported close to reaching an 11th-hour compromise agreement on a package of constitutional amendments designed to keep predominantly French-speaking Quebec from seceding and the 123-year-old Canadian confederation from breaking up.

However, after more than 12 hours of negotiating, the provincial and federal leaders were unable to iron out still-existing differences and recessed at 10 p.m. for another round of talks Friday.

A potential impasse arose when the premier of Quebec, Robert Bourassa, announced that he would no longer sit at the table if the talks "touch directly or indirectly" on altering a previous commitment to recognize Quebec as a "distinct society."

"So far as Quebec is concerned, that question has been settled," Bourassa said, suggesting that the negotiations could collapse over the issue.

The fragile compromise package, the subject of five days of marathon negotiations in Ottawa, would preserve a set of constitutional amendments initialed by federal and provincial leaders in April 1987 at Meech Lake, Quebec. It would add provisions designed to satisfy critics of the original accord, who had rescinded or withheld their provinces' ratification.

The Meech Lake accord will expire unless it is ratified by all 10 provinces by June 23. The accord met Quebec's five "minimal" conditions for signing Canada's 1982 constitution. It recognized the province's distinct character, gave the province three of the nine judges on the Supreme Court bench, guaranteed Quebec a proportionate share of immigrants entering the country and allowed provinces to opt out of shared-cost federal programs.

The new compromise package that has been under discussion, according to sources close to the negotiations, calls for the ratification of the Meech Lake accord by the June 23 deadline, accompanied by a commitment to consider a series of add-on provisions later.

One of these would retain the clause in the Meech Lake amendments designating Quebec a "distinct society" -- one of the demands that the province's leaders said was nonnegotiable. But the agreement would incorporate a "Canada clause" to broaden the definition of the Canadian identity and explicitly recognize the multicultural character of the country, its federal nature and the equality of provinces.

The clause, in effect, would recognize that a number of distinct societies exist in Canada besides that in Quebec. It has been described as a "We the peoples" clause.

As part of a "priority agenda," the agreement reportedly would provide for an overhaul of the appointed, and largely ineffectual, Senate chamber of Parliament, which is widely regarded by Canadians as a bastion of patronage with few powers other than to stall bills adopted by the elected House of Commons.

Under the proposed agreement, a national commission of parliamentary and provincial leaders would conduct hearings throughout the country on Senate reform. If the provinces and federal government were unable to agree on a way to correct the imbalance of seats in the 104-member Senate, smaller provinces would automatically get up to four more seats each, while the populous provinces of Ontario and Quebec would retain 24 seats each.

The new constitutional package is a hybrid of compromises that seem designed to allow Quebec Premier Bourassa to reassure his constituency that the 1987 Meech Lake accord was not changed, while at the same time allowing the holdout premiers of Manitoba and Newfoundland to go home saying it was.

Earlier tonight, the leader of the separatist Parti Quebecois, Jacques Parizeau, warned Bourassa that if he allowed any tampering with the "distinct society" clause, he would incur the wrath of "seven million . . . Quebeckers."

Also, Manitoba Premier Gary Filmon, one of the Meech Lake holdouts, was reported to have told Bourassa that he would have to allow a clause qualifying the "distinct society" provision to appear in the constitution.

Filmon tonight said the talks had reached a "critical stage."

For more than six months, the debate over Meech Lake has preoccupied and divided the country more than any of the previous constitutional crises that periodically have flared -- crises that always seemed to dissolve in what Canadians like to characterize as a national trait of compromise and accommodation.

The debate set regions of the vast subcontinent against one another and fueled long-simmering antagonisms between English and French speakers, in an atmosphere of mean-spiritedness that veteran political observers said was unparalleled in recent history.

It was marked by such ugly scenes as English-speaking radicals in Ontario wiping their feet on Quebec's fleur-de-lis provincial flag for the benefit of television cameras, and separatist French Canadians publicly warning Anglophones that they would soon need passports to visit the province.

The acrimony, besides deeply troubling a country that prides itself on tolerance and its multicultural character, also sent reverberations through the money market, driving down the value of the Canadian dollar, sending interest rates up and causing nervousness among foreign investors. The dispute appeared to indicate that centrifugal forces of regionalism are growing under a policy of decentralization that was intended to promote national unity.