Conoco Inc. the most hotly pursued takeover candidate in Wall Street history, yesterday said it opposed the $7.8 billion bid made by Mobil Co. last week and said it still supports the $7.4 billion offer from E.I. duPont de Nemours & Co.

Conco Chairman Ralph E. Bailey said that the board of directors, in unanimously opposing the Mobil offer, also authorized the company to go to court to try to block Mobil's offer on the grounds that it would violate antitrust laws.

Du Pont has promised to retain Conoco's top management if the two companies are merged. Mobil has not made such a promise.

Mobil, with annual sales of more than $59 billion last year, is the second-largest oil company in the country. Conoco, at $18.3 billion is the ninth biggest.

Mobil said last week that its lawyers had determined there would be no antitrust violation in a merged Mobil and Conoco. The federal government will review the proposed Mobil takeover, and is reviewing as well the Du Pont offer and a $3.77 billion bid for 51 percent of Conco's stock made by the big Canadian distiller, Seagram Co.

"If Mobil is permitted to acquire Conoco -- which competes vigorously and broadly with Mobil in all phases of the petroleum business -- a major restructuring of the U.S. industry is likely to result, with substantial energy enterprises like Conoco disappearing as vigorous competitors from the market."

Meanwhile, Gulf Oil Corp. joined the long line of oil companies arranging huge lines to credit to, as the company put it, "act on opportunities which may be present in today's business environment."

Gulf, the nation's fifth-largest oil company, said it is negotiating for a $5 billion line of credit. It already has in hand a $1 billion line, which it expects to use for "current financial requirements" including previously announced capital-spending plans. Bankers Trust Co. of New York and National Westminister Bank of London head the banking syndicate that would offer the credit.

Spokesman for Gulf refused to say whether the company has any plans for acquistions or mergers. "It's like lining up a convertible to take a girl to the beach," one quipped.

Most of the liens of credit have been set up to finance the bidding war for Conoco. Mobil arranged a $6 billion credit line for its $90-a-share bid for Conoco.Earlier, Texaco, number three among U.S. oil companies, negotiated a then-record $5.5 billion line, fueling speculation it, too, would enter the contest for Conoco. As of yesterday, Texaco still had made no announcement about its intentions.

Even far smaller Marathon Oil Co. has set up a $5 billion credit line. In that case, as with Penzoil, which has arranged a $2.5 billion line, analysts said the moves were largely defensive. With such financing, the companies might outbid another firm's offer for their publicly held stock or take other actions to prevent a takeover.

Marathon, Penzoil, Kerr-McGee Corp., Phillips Petroleum Co. and Union Oil Co. of California are all on the list of potential takeover targets. Big as it is, conceivably even Gulf could be the target of an acquisition move, too.

As in Conoco's case, Gulf's oil, gas and coal reserves are worth considerably more than its current stock price, which was $36.63 at yesterday's close. That is between one-third and one-fourth of what company officials estimate Gulf's asset value to be.

Analysts said yesterday's step could be primarily defensive. Gulf last week said it plans to buy up to 10 million of its own shares, which would use up more than one-third of the $1 billion credit line it definitely plans to use.

With the large amount of cash available, Gulf could make a tender offer for its own shares if necessary to fight off a tender offer from outside.

Such an offer might be an effort to obtain control of enough stock to force the sale of Gulf Canada.Dome Petroleum Co. Ltd., a Canadian company, used such a tactic earlier this year to force Conoco to sell its Canadian subsidiary.

Sal Ilacqua, an analyst with L.F. Rothschild, Unterberg, Towbin, investment bankers, said that since Seagram apparently "are beaten on Conoco, they might well try another." With Texaco, Gulf and Mobil -- if it should lose out on Conoco, too -- having all that credit in hand, they could be "the white knights waiting in the wings" to do battle with Seagram.

Ilacqua said he had been expecting a spate of takeovers and mergers "a couple of years down the road," but that the rapidity with which it is happening now "is flooring me.