President Reagan took the offensive yesterday for the first time against the perception that he is presiding over a leaderless administration in which the key decisions are being made on his behalf by the White House staff.

Appearing in the White House briefing room for what spokesman Larry Speakes described as "a mini-press conference" with carefully controlled ground rules, the president gave good-humored expression to feelings that aides said he had expressed with force and anger in private.

"There has been such disarray approaching chaos in the press corps, with regard to the subject of arms control, that I thought, before you unraveled into complete disorder, that maybe we should straighten out the entire subject," Reagan said, with a slight smile, as he read a statement intended as a parody of press accounts describing the president as detached from some of the central policies of his administration.

Reagan's attempt to demonstrate that he is firmly in control and fully knowledgeable about the policies of his administration was marred by his reference to Paul H. Nitze, his chief arms control negotiator, as "Ed Nitze."

Some aides also were concerned that Reagan would appear overly defensive about stories critical of his leadership. One recalled the time in 1974 when then-Sen. William L. Scott (R-Va.) called a press conference to denounce an article in an obscure magazine that described him as "the dumbest congressman of them all."

But Reagan gave a spirited defense of his leadership, proclaiming that he makes the decisions in his administration, and told aides afterward that he had enjoyed the session.

Speakes said Reagan plans to hold similar "impromptu, mini-press conferences" on a weekly basis. But they will be carefully limited in form and length.

Before the president entered the briefing room Speakes spelled out ground rules that required reporters to raise their hands and exhaust questions on arms control before proceeding to other subjects.

However, Reagan's own reference to "disarray" opened up this issue, and most of the questions wandered back and forth between the subjects of arms control and presidential leadership.

The form of the mini-press conference, patterned after the ground rules of Sacramento news conferences when Reagan was governor of California, is intended to be protective of the president. Tradition in Sacramento requires that reporters ask all the questions they have on one subject before proceeding to another.

But those press conferences were 30 minutes long, and Speakes said yesterday that the sessions planned here by Reagan will be 10 minutes in length. This will in most cases limit reporters to one or two subjects, on which Reagan will be carefully briefed beforehand.

Aides are confident they can accurately predict most of the questions in this format. The emphasis is directed at the television networks, which carried the president live yesterday. Of the 14 questions Reagan answered in 13 minutes, 10 were asked by network television correspondents.

Reagan and some of his key aides have long been concerned about reports that the president isn't in control of his administration. Yesterday, responding to questions about stories on this subject in The Washington Post, Speakes described them as "untrue, foolishness and misconceived journalistic spinoffs."

Ever since he first sought office in 1966, Reagan has been sensitive about accusations that he is uninformed, in part because this was a principal theme of his campaign opponents in California.

When Reagan ran into periods of trouble in his early campaigns, as well as in his presidential races, he blamed the press for focusing too much on his gaffes. In 1980, when several stories at the same time focused on his slip-ups and misstatements, Reagan accused the press of "journalistic incest."

Since Reagan arrived at the White House, staff sensitivity about mishaps has led to long periods of presidential inaccessibility. Reagan has been conspicuously more accessible since early December, and there is a belief among his advisers that they can show the president to best advantage in the short, limited-subject format.

Reagan, who had been briefed on arms control, appeared anxious yesterday to stick to that issue, saying at one point, when a reporter persisted in asking about his staff, that "we are getting too far away from the general subject."

The president acknowledged that staff and Cabinet members and Republican senators, including his friend Sen. Paul Laxalt (R-Nev.), had been quoted as advocating positions different from his own on budget and defense issues. He contended, however, this represented not disarray but a willingness to consider many conflicting views in making decisions.

"It is true that I ask, and want to hear, differing viewpoints on things," Reagan said. "But then, I make the decisions. And this has been working very well. And we have had a very heavy agenda for the last few weeks. We have been working long hours on a number of things that are before us here."