WASHINGTON ? Hundreds of television stations across the United States pulled the plug on Friday on analog broadcasts and switched to a digital-only signal.

Despite an intensive campaign by the Federal Communications Commission (FCC) nearly three million US homes were expected to be left with a blank screen when the transition was completed at midnight Friday (0400 GMT Saturday).

More than 600 stations ? one-third of the nearly 1,800 full-power television stations in the United States ? have already ended analog broadcasts and the remainder had until the midnight deadline to do so.

Major TV networks such as ABC, CBS, Fox and NBC were among those making the switch on Friday.

Acting FCC chairman Michael Copps hailed the move to all-digital broadcasting as an "historic" event.

"Today's historic transition to digital TV is an important step forward in US broadcasting, offering consumers access to more free over-the-air programming as well as higher quality pictures and sound," he said.

"The transition also frees up valuable airwaves for emergency communications and advanced wireless services," Copps said in a statement.

"At the same time, I recognize the great challenge that today's switch presents for many consumers," he added.

"Even though the overwhelming majority of households are DTV-ready, we are fully committed to helping those who have yet to join the digital television age both today and in the days to come."

The Neilsen Co. said Wednesday in its final update before the switch that 2.8 million US households ? 2.5 percent of the television market ? were "completely unready" for the transition, down from 5.8 million in February.

"Younger, African-American and Hispanic homes are disproportionately unready, while the elderly are the most ready," according to Nielsen.

The transition was originally scheduled for February 17 but was delayed at the last-minute by Congress and President Barack Obama in a bid to prepare more homes for the switch.

The government has been providing Americans who rely on over-the-air signals with a $40 coupon to defray the cost of buying a digital converter box.

The FCC has issued 59 million coupons for the converter boxes and will continue to supply them until July 31.