The Federal Trade Commission voted yesterday to oppose two mergers that would have given Coca-Cola Co., which is seeking to acquire Dr Pepper Co., and PepsiCo Inc., which wants to buy Seven-Up Co., 81 percent of the soft-drink market.

FTC commissioners voted unanimously to seek injunctions against the two mergers, saying that the proposed acquisitions could reduce competition in the distribution and sale of carbonated soft drinks.

The FTC, which cannot itself block the proposed mergers, directed its staff to seek preliminary injunctions next week in federal court to halt the transactions. Their combined cost is $850 million.

At about the same time yesterday, a federal judge in Columbus, Ga., granted a request by competitor Royal Crown Cola Co. for a temporary restraining order to prevent the mergers.

The FTC votes, which followed a lengthy discussion, were 4 to 0, with Commissioner Mary Azcuenaga not voting. Her husband, an attorney, works for a firm that represents Dr Pepper.

Coca-Cola and Pepsi are expected to try to oppose the attempts to bar the mergers.

Critics of the Reagan administration's stance on antitrust yesterday said they were pleased by the FTC's action.

"These mergers were so outrageous and so harmful to consumers that even this administration realized it had to oppose them," said Sen. Howard Metzenbaum (D-Ohio).

The mergers also would have exceeded the statistical guidelines used by the FTC and the Justice Department's antitrust division to determine whether business combinations are likely to harm competition.

Coca-Cola's share of the soft-drink market would increase from 38 percent to approximately 45 percent if it were allowed to acquire Dr Pepper, while Pepsi's share would rise from 29 percent to about 36 percent if it acquired Seven-Up. Coca-Cola is the nation's No. 1 soft-drink manufacturer, and Dr Pepper is No. 4. The merger of Pepsi and Seven-Up would combine the second- and third-ranking soft-drink makers.

Coca-Cola declined to comment. A spokesman for PepsiCo said the company was "obviously disappointed." Pepsi is considering its options, he added.

Pepsi had moved first by arranging to buy Seven-Up from Philip Morris Cos. for $380 million in January. Coca-Cola soon followed with its bid to buy Dr Pepper for $470 million from an investment group headed by Forstmann Little & Co.

Jesse Meyers, publisher of the trade publication Beverage Digest, said that Coke's move may have been aimed, in part, at raising the concentration in the soft-drink industry so high that the FTC would have to oppose both deals. Pepsi was particularly interested in Seven-Up's foreign operations, and Coca-Cola would not welcome tougher competition overseas from Pepsi, Meyers said.

If the proposed mergers fail, "It will be mostly business as usual for Coke and Pepsi, with a couple of exceptions," said Emanuel Goldman, an industry analyst with Montgomery Securities in San Francisco.

Goldman also said that Pepsi probably would be able to retain Seven-Up's overseas business, adding 20 percent to its overseas volume. If the FTC is victorious in court and Pepsi begins searching for another buyer for Seven-Up, as it is required to do under the terms of its deal with Philip Morris, it may turn to Royal Crown, he said.

The opposition to both mergers was led by Royal Crown Cola Co., which would be the largest remaining independent soft-drink company if the mergers occurred. Royal Crown has about 5 percent of soft-drink sales, while Seven-Up and Dr Pepper each have about 7 percent.

If the mergers are blocked, a new realignment of the industry may follow, said Meyers, with Royal Crown perhaps teaming up with Cadbury Schweppes PLC of Britain, which wants to buy Canada Dry and Sunkist brands from RJR Nabisco.

Conservative economists had hoped the FTC would use the soft-drink decisions to dramatize the Reagan administration's new approach to mergers and antitrust enforcement. That approach emphasizes close assessment of markets rather than reliance on numerical analysis of companies' market shares.

A close look at the soft-drink market shows more competitive strength than the dominant positions of Coke and Pepsi might suggest, said Bob Tollison, director of the Center for Public Choice at George Mason University, who assisted in preparing Pepsi's case before the FTC.