Worried investors sent North American stocks lower Thursday after a series of explosions rocked London.

Shortly after Thursday’s open, the Dow Jones industrial average was down 28.16 points, the Nasdaq composite index was off 18.35 points and the S&P 500 was down 3.08 points.

In Toronto, the S&P/TSX composite index was down 50.06 points, or 0.5%, at 10,061.83.

All 10 of the TSX index’s main groups were lower, led by a 0.9% drop in the energy group.

A series of coordinated explosions rocked London subway stations and ripped apart a double-decker bus at the morning rush hour Thursday, killing at least two and injuring many more, and prompting officials to shut down the entire underground transport network.

British Home Secretary Charles Clarke has confirmed four blasts.

Police have confirmed two deaths and 54 injured in the subway blasts, but hospitals in the city say there may be more than 100 injured. CNN and Reuters are reporting as many as 12 people were killed.

U.K. Prime Minister Tony Blair said the “barbaric” terrorist attacks were designed to coincide with the opening of the Group of Eight summit.

The explosions sent stocks plummeting in Europe. Insurance and travel stocks fell sharply, and the British pound also fell. The FTSE 100 dropped 207.54 points to 5,022.10 by early afternoon. However, market indices have now risen from their session lows.

The FTSE-100 was last down 2% at 5,123. The DAX was down 2.3% at 4511, while in Paris, the CAC 40 dropped 2.5% to 4,175.. Exchanges elsewhere in Europe were also down.

The Bank of England and the European Central Bank both left key interest rates unchanged at policy-making meetings today.

World oil prices retreated on in the wake of the London explosions.

The price light sweet crude for August delivery was trading at US$59.90 US per barrel down $1.38.

In Canadian economic news, Statistic Canada said today that the overall value of building permits fell for the second straight month in May, as industrial and commercial intentions declined and the value of housing permits remained virtually unchanged.

StatsCan said municipalities issued permits worth $5.0 billion, down 2.1% from April.

South of the border, n economic news, the U.S. Labor Department said initial jobless claims rose by 7,000 to 319,000 for the week ending July 2.

Economists had called for jobless claims to have increased by 10,000 to a level of 320,000 in the week.

U.S. aluminum giant Alcoa kicks off the second-quarter earnings season late today when it reports after the stock market closes.

On Wednesday, Toronto stocks ended higher, while New York markets slumped, as oil prices rose amid concerns over crude production and refining capacity.

The S&P/TSX composite index finished up 36.76 points, or 0.36%, to close 10,111.89.

The junior S&P/TSX venture exchange finished up 1.65, or 0.10%, to close 1,725.54

In New York, the Dow Jones industrial average lost 101.12, or 0.97% to close 10,270.68. The tech-heavy Nasdaq composite index declined 10.10, or 0.49%, to 2,068.65, while the S&P 500 dropped 10.05 points, or 0.83%, to 1,194.94.

The increase in oil prices offset positive news from the American non-manufacturing sector. The U.S. Institute for Supply Management reported its index of business activity stood at 62.2 for June, above economists’ expectations.