Updated from 2:35 p.m. EDT

More than 24 hours after it began, the massive power outage that hit several northeastern states and Canada continued to plague parts of the country, despite efforts to restore power and a sense of normalcy.

After opening as scheduled Friday morning, the

New York Stock Exchange

and the

Nasdaq

completed a full day of trading, but the volume on both exchanges was less than half of daily average. Meanwhile, the

American Stock Exchange

was open for just 30 minutes Friday afternoon and the

New York Mercantile Exchange

planned to close early.

While power was restored in many of the areas affected, parts of New York City, Detroit and Cleveland were still without power Friday afternoon. While New York Mayor Michael Bloomberg said Friday morning he expected power to be fully restored by the end of the day, his representatives later backed away from that timetable.

Although the Mayor's office still "hopes" to have power fully restored by the end of the day, "you never know what's going to happen," said Jordan Barowitz, a spokesman for Mayor Bloomberg.

"We believe we are getting very close. The folks at (Consolidated Editson) are working hard to get everything back online," Barowitz said. "But we're in a situation where we have zero excess capacity. If we have any problem, it could be a serious setback.

"There's little room for error at this stage of the game."

The outage knocked out electricity to about half of New York state on Thursday, but by noon EDT on Friday, power had been restored to essentially all of upstate New York, said Dennis Michalski, a spokesman for the State Emergency Management Office. Additionally, power had been restored to about 80% of Long Island, he said.

Michalski did not know when power would be complete restored to all areas of New York.

In New Jersey, the blackout hit four of the state's 21 counties, said Trooper Stephen Jones, a spokesman for the New Jersey state police, which coordinates emergency response in the state. But by 2:45 p.m. EDT on Friday, power had been fully restored to all of the customers affected by the outage, Jones said.

New Jersey hasn't yet compiled data from its counties to come up with an estimate of the economic cost of the outage, Jones said. Jones did not know how many people were affected by the outage at its peak.

The blackout, which started at about 4:11 p.m. EDT on Thursday, hit areas ranging from Detroit and Cleveland in the west to Toronto in the north and as far south as southern New Jersey. Although initial indications were that the outage was caused an overload of a power grid operated by Syracuse, NY-based Niagra Mohawk, officials backed away from that assessment on Friday.

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In a press conference, New York Mayor Michael Bloomberg said the problem started in Canada. Some kind of "event" happened in Canada that caused Canadian energy operators to demand power from New York's power grid. Under a power sharing arrangement between Canada and the United States, the two countries share power to help relieve peak demand.

The demand on the New York grid was more than it could accommodate and it shut down, Bloomberg said.

The event that caused the blackouts happened somewhere in the vicinity of Lake Erie, said Bryan Lee, a spokesman for the Federal Energy Regulatory Commission. Energy officials still have no idea what caused the problem, but do not think it was the result of terrorism, Lee said.

By Friday afternoon, power had been restored to 60% of New York City, including all of Staten Island and to Lower Manhattan, said Barowitz. However, areas of Manhattan and "large pockets" in the Bronx and Queens were still without power, he said.

As a result of the outage, many office buildings in New York were closed on Friday, Barowitz said. Meanwhile, the subway system, which has been shut down since the blackout began, is not expected to be back up and running until six to 10 hours after power is fully restored to the city, he said.

On the financial markets, about 562 million shares were traded on the New York Stock Exchange on Friday, compared with a normal daily volume of about 1.3 billion shares. On the Nasdaq, some 700 million shares traded hands; recent share volume has exceeded 1.4 billion shares a day.

Meanwhile, the American Stock Exchange, which originally forecast that it wouldn't open on Friday, resumed trading around 3:45 p.m. EDT. The market closed trading at 4:15 p.m. EDT.

The city does not yet have an estimate of the economic cost of the blackout, Barowitz said.

New York wasn't the only area trying to get back to business. In Ohio, for instance, power was fully restored by Friday morning to 11 of the 15 counties that were affected by the outage. However, in the county where Cleveland is located and in three other counties, only 70% of power had been restored, said Rob Glenn, a spokesman for the state's emergency management agency. The state hoped to fully restore power to the remaining counties by the end of the day, Glenn said.

At the peak of the outage, power was out for 1 million to 1.5 million people in Ohio, Glenn said. However, the agency does not yet have an estimate of the economic cost to the state of the blackout, he said.

Things were a little darker in Michigan, where 1.2 million people remain without power. Although that's down from a peak of 2.1 million people on Thursday, the main area that continues to be without power is the Detroit metropolitan area, said Mark Wesley, a spokesman for the emergency management division of the Michigan State Police department. The energy company serving Detroit doesn't expect to fully restore power to the area until late Sunday, Wesley said.

The emergency agency does not yet have an estimate of the economic impact of the outage to the state, Wesley said.

Power's Slow Return

At the peak of the outage, 61,800 megawatts of electricity were taken offline, said Tim Gallagher, a spokesman for the North American Electric Reliability Council (NERC). Each megawatt is enough to power 1,000 homes, Gallagher said, implying that up to 62 million homes could have been affected by the blackout.

Areas affected included Ohio, Michigan, New York, New Jersey, Massachusetts and Connecticut in the United States and Canadian provinces Ontario and Quebec, Gallagher said.

By the early evening on Friday, about two-thirds of the lost power had been restored, Gallagher said. But parts of Ontario, New York, New England and Michigan were still lacking power, he said. Gallagher had no precise estimate on when power would be fully restored to all areas.

However, Ontario might have to wait until sometime next week to have all of its power back online, he said. Of 11 nuclear plants that went offline during the outage, only four have resumed operations, he said. The remaining power plants will take more time to come back online, because of the way they were built and the fact that they were shut down so quickly, he said.

Meanwhile, the problem that power producers are facing in Detroit is a damaged generator, Gallagher said. As the outage occurred, some of the components of the generator overheated, he said. Those will have to be repaired before power production can resume, he said.

NERC, which works with regional power suppliers in the United States, Canada and Mexico in an effort to prevent outages such as the one that started yesterday, has not yet determined what caused the blackout or where it started, Gallagher said. Prior to the outage, a number of transmission lines overloaded in the Cleveland area, but NERC doesn't know yet whether those incidents were related to the wider blackout, he said.

"We've not determined what the root cause is," he said. "We're putting the pieces together."

Despite the widespread nature of the outage, the power system worked as designed, Gallagher said. If not for the safety precautions that were put in place, the outage would have been even more extensive that it was, potentially blacking out all of the eastern United States, he said.