SAN FRANCISCO (AP) — Transit officials said Friday that they blocked cellphone reception in San Francisco train stations for three hours to disrupt planned demonstrations over a police shooting.

Officials with the Bay Area Rapid Transit system, better known as BART, said they turned off electricity to cellular towers in four stations from 4 p.m. to 7 p.m. Thursday. The move was made after BART learned that protesters planned to use mobile devices to coordinate a demonstration on train platforms.

"A civil disturbance during commute times at busy downtown San Francisco stations could lead to platform overcrowding and unsafe conditions for BART customers, employees and demonstrators," BART officials said in a prepared statement.

The statement noted that it's illegal to demonstrate on the platform or aboard the trains. BART said it has set aside special areas for demonstrations.

The American Civil Liberties Union questioned the tactic.

"Shutting down access to mobile phones is the wrong response to political protests," the ACLU's Rebecca Farmer said in a blog post.

The Electronic Frontier Foundation said on its website that "BART officials are showing themselves to be of a mind with the former president of Egypt, Hosni Mubarak." Mubarak's regime cut Internet and cellphone services in the country for days early this year while trying to squelch protests demanding an end to his authoritarian rule.

BART officials were confident the cellphone disruptions were legal. The demonstration planned Thursday failed to develop.

"We had a commute that was safe and without disruption," said BART spokesman Jim Allison.

The demonstrators were protesting the July 3 shooting of Charles Blair Hill by BART police who claimed Hill came at them with a knife.

A July 11 demonstration disrupted service during the rush-hour commute, prompting the closing of BART's Civic Center station. Several arrests were made.

NEW YORK (MarketWatch) -- U.S. stocks plummeted Thursday, with the benchmark indexes down about 4% or more, on worries about Europe and the global economy.

“Market sentiment continues to deteriorate amid concerns about the euro-zone banking sector,” Marc Chandler, global head of currency strategy at Brown Brothers Harriman & Co., wrote in a note.

U.S. economic reports did not help in allaying concerns, as the number of Americans filing new claims for jobless benefits rose last week; consumer prices climbed 0.5% in July; and the Philadelphia Federal Reserve said factory activity in the region fell sharply in August.

Read more on jobless claims.

Read more on consumer prices.

Read more on Philly Fed.

Morgan Stanley reduced its forecast for global growth, calling Europe’s policy answer to its sovereign debt crisis insufficient; The Wall Street Journal cited people familiar with the matter in saying U.S. regulators had intensified scrutiny of the U.S. arms of European banks, and Sweden’s financial regulator said the nation’s lending institutions must take further steps to ready for Europe’s debt crisis to darken further.

See WSJ story on Fed scrutiny of European bank arms.

After falling as much as 528 points, the Dow Jones Industrial Average (DJIA - News) was lately down 423 points, or 3.7%, to 10,987.21, with all of its 30 components shedding value, led by Bank of America Corp. (NYSE:BAC - News) , off 6.8%.

The Standard & Poor’s 500 Index (TSXV:SPX.V - News) declined 47.97 points, or 4%, to 1,145.92, with industrials slammed the hardest among its 10 industry groups, all of which were losing ground.

The Nasdaq Composite Index (COMP - News) dropped 110.94 points to 2,400.54.

Decliners rapidly outpaced advancers on the New York Stock Exchange, where 349 million shares had traded by 10:30 a.m.