Threats prompted brief evacuations of a Toronto subway station and a newspaper office. (Representative)

A rash of bomb threats were sent via email on Thursday to dozens of businesses and public buildings across the United States and Canada demanding payment in cryptocurrency, but none of the threats was immediately found to be credible, law enforcement officials said.

Starting shortly before 2 pm EST (1900 GMT), police departments in major US cities coast to coast began reporting on Twitter that numerous local businesses had received the emails - awkwardly worded threats to set off a bomb unless a bitcoin payment of $20,000 were received.

More than two hours into the security scare, no actual explosives had been found, authorities said. But the threats prompted brief evacuations of a Toronto subway station and a newspaper office in Raleigh, North Carolina. Some public schools and business also were evacuated as a precaution.

Among the cities where bomb threats were reported by authorities on official Twitter accounts were New York, Detroit, San Francisco, Oklahoma City, Denver, Ottawa, and Calgary, Alberta.

Police at the University of Wisconsin in Madison tweeted an image taken of one email threat found to be circulating that said in part: "Good day. There is an explosive device (lead azide) in the building where your company is conducted. It is assembled according to my guide. It is compact and it is covered up very carefully. It can not damage the structure of the building, but in case of its explosion you will get many wounded people."

Police in Cedar Rapids, Iowa, reported similar email threats received by several businesses there but had found "no credible evidence any of these emails are authentic."

The FBI has launched a query into the matter but the authenticity of the latest batch was not immediately confirmed, a law enforcement official told Reuters.

"We are aware of threats being made in cities across the country," Rukelt Dalberis, an FBI spokesman in Los Angeles, told Reuters separately. "We remain in touch with our law enforcement partners. We encourage the public to remain vigilant and report suspicious activities that could represent a threat."

Multiple US law enforcement sources told Reuters that no actual explosives had surfaced in connection with any of the threats within the first couple of hours of the scare.

A similar wave of emailed hoax bomb threats in December 2015 prompted officials in Los Angeles to close the city's public school system, a move that national law enforcement officials later criticized as an overreaction.

That threat came two weeks after a married couple inspired by Islamic State killed 14 people at a California county office building in a shooting rampage.

A teenager with dual Israeli-US citizenship was arrested in Israel in March 2017 for making bomb threats to more than 100 Jewish organizations and Jewish community centres (JCCs) in dozens of US states over several months.