A day after his company arranged a $4 billion credit line as a possible takeover defense, Cities Service Co. president C. J. Waidelich warned the Reagan administration yesterday "to look very carefully" at the antitrust implications of Mobil Corp.'s proposed takeover of Conoco Inc.

Waidelich's remarks on the bidding war for Conoco, as well as criticism of the Canadian government's National Energy Program, were part of a wide-ranging discussion yesterday at a luncheon with editors and reporters from The Washington Post.

Waidelich qualified his criticism of the proposed Mobil-Conoco merger, saying "I'm not passing judgement on Mobil's offer." He added that "I'm not an antitrust lawyer. 'm not down here waving the flag saying that Mobil cannot acquire Conoco."

But the head of the nation's 19th-largest oil company offered strong antitrust objections to the proposed takeover of the ninth-largest oil firm by the company that ranks number two after Exxon Corp. "If by approving one such transaction, do you then send out a signal which creates a trend to other mergers?"

Such a trend, he said, could reduce competition in the oil industry by eliminating medium-sized oil companies, Waidelich said. If the Mobil-Conoco merger is approved, he said, "then I think companies the size of Cities Service must be very concerned." An initial indicator of the administration's handling of the takeover may come today, which is the deadline for the Justice Department's request for information.

The Canadian government's energy policies are "unfair," Waidelich said, especially the treatment of Cities Service's interest in a massive tar sands project in the Canadian west. "We were encouraged to go ahead with our syncrude project in the tar sands and build this project," he said.He objected to additional Canadian federal taxes and to what he termed "the actual abrogation" of an agreement relating to prices for the project's products.

Waidelich also objected that Canadian ownership of the nation's energy resources have "devalued" Cities Service's Canadian assets, including the syncrude project and rights to large tar sands deposits. Cities Service filed suit against the Canadian Nu West Group Ltd., which last spring bought 7.2 percent of Cities Service's shares and then offered to exchange them for the American company's Canadian holdings.

Cities Service claims that Nu West could not have made such a large purchase without Canadian laws that permit companies to borrow the entire amount needed for a securities purchase. In a related development, a Senate Banking, Housing and Urban Affairs subcommittee yesterday approved a bill that would restrict the amount foreign firms may borrow to invest in U.S. corporations.

The measure -- which is similar to a bill already adopted by a House subcommittee -- would allow foreign investors who want to buy 5 percent or more of a U.S. corporation to borrow no more than 50 percent of the purchase price of the stock. A similar "margin" limit now applies to U.S. investors.