At least two people are dead and more than 23 injured, according to official figures, following a series of simultaneous bomb blasts near the finish line of the Boston Marathon.
The city is in lockdown and security has been tightened in Washington and New York, with Boston Airport shut down and a no-fly zone imposed over the city.
The Boston Globe reports more than 100 people have been injured as a result of the explosions, which took place at around 2.50pm local time.
It is currently unclear whether a fire at the JFK Library is connected to the Boston Marathon incident.
“We still do not know who did this, or why, and people shouldn’t jump to conclusions before we have all the facts,” says US President Barack Obama.
“But, make no mistake, we will get to the bottom of this and we will find out who did this, we will find out why they did this.
“Any responsible individuals, any responsible groups will feel the full weight of justice.”
Foreign Minister Bob Carr says Australian officials are working to determine whether any Australians are among those admitted to hospital.
Gold price crash
The price of gold has plunged by more than 9%, with a single-day loss of more than $US100 per ounce, marking the biggest single day loss for the commodity since 1983.
The fall in gold price has triggered a sell-off of other precious metals, including silver, platinum and palladium.
The news has triggered speculation the bull market in gold could be over, ending 12 consecutive years of gains.
Chinese growth misses expectations
China has reported a first-quarter gross domestic product growth rate of 7.7%, falling below the 7.9% rate reported in the preceding quarter.
The news has disappointed some analysts, who had looked to a return to strong growth in China, although the quarter is notorious for “red herrings” due to the disruption of the Chinese New Year.
“Overall, the Chinese economy had a smooth start in 2013, but many uncertainties, at home and abroad, still persist and make the overall situation quite complicated,” said Premier Li Keqiang.
The Dow Jones Industrial Average is down 1.8% to 14,599.20. The Aussie dollar is down to US103.09 cents.