NEW YORK, JUNE 29 -- NBC management got off to a slightly rocky start in the production studios today after 2,800 technicians, producers, writers and editors in six cities walked out in a dispute over job security.

With managers behind the cameras and in the control room, NBC's "Today" show was plagued by technical foul-ups, including fumbled microphones, sloppy camera work and sound lapses that at one point rendered coanchor Bryant Gumbel inaudible.

Still, the top-ranked network continued broadcasting without interruption, and spokeswoman McClain Ramsey denied there had been any "major problems." "To the contrary," she said, "everything has been quite smooth." NBC said it had trained management and nonunion employes to do the strikers' work.

Members of the National Association of Broadcast Employees and Technicians struck the network at 12:01 a.m. eastern time after last-gasp negotiations failed and NBC put its final contract offer into effect.

Picket lines went up outside NBC headquarters in New York and at network facilities in Burbank, Calif., Chicago, Cleveland, San Francisco and Washington.

No violence or vandalism was reported at the start of the third strike against a network this year.

"NBC Nightly News" anchor Tom Brokaw called the strike "a tragedy. It's very painful because we've got our friends on the street. The morale of the place, well, everybody's doing their best."

In Washington, NBC provided a shuttle bus for non-NABET employes who wanted to enter the network complex without walking past pickets.

In Los Angeles, union crews from non-NBC stations refused to staff a prosecutor's news conference because the NBC affiliate, KNBC, had sent nonunion replacement workers. As a result, two news conferences were held.

The disputed two-year contract offered workers wage hikes of 3.6 percent and 4.6 percent, but the negotiations stalled over job security. NBC wants the right to hire free-lance workers for jobs now done by permanent employes.

"We're out there because the company put us out there," said Arthur Kent, president of NABET Local 11. "They wanted this strike."

However, NBC noted that the National Labor Relations Board last week dismissed seven charges of unfair labor practice filed by the union and accused the union's leadership of ignoring members.

"We believe that if union members had been given the opportunity of working under the new contract, they would agree with NBC that it is a fair, reasonable and balanced agreement for the company, the union and their employes," said a network statement read by Ramsey.

A 3 1/2-hour session called by a federal mediator Sunday failed to produce an agreement or push back the midnight deadline. It was only the second session since the contract expired March 31.

No further sessions were planned. Kent said he expected it would be at least a week before the mediator called the sides back together.

Both sides said they were hoping for a short strike but were prepared for a long one.

"It doesn't look good because we're dealing with ... GE, and their history isn't that good," said Barbara Nellis, a graphics engineer picketing in front of NBC headquarters.

The General Electric Co. took over RCA, NBC's parent company, last year.

"We've had a relationship with NBC for 50 years," said John Krieger, a union official in Washington. "Now GE has taken control and we are being subjected to a strike we don't want."

The strike affects approximately one-third of NBC's work force, including camera and videotape operators, engineers and electronic maintenance workers, writers, editors and production assistants.

Union members insisted that the quality of programming would suffer even though NBC says nonunion personnel have been trained to perform the jobs under NABET jurisdiction.

"Just common sense tells you there's going to be glitches, there's going to be screw-ups," said Kent. "The viewing public will not get the production that NBC has traditionally given them. And, even more important, the advertisers -- the clients -- won't get the results they're used to."

In the spring, 575 CBS and ABC editorial workers struck for nearly two months, also over job security. In those disputes, the networks said they gained flexibility in hiring and firing while the unions claimed partial victories.

In addition to the NBC network, the strike affects network-owned stations WRC-TV (Channel 4) and WKYS-FM (93.9) in Washington, as well as KNBC-TV in Burbank; WKQX-FM, WMAQ-AM and WMAQ-TV in Chicago; WKYC-TV, Cleveland; WNBC-AM, WYNY-FM and WNBC-TV, New York; and KNBR-AM and KYUU-FM, San Francisco.