Two threatening letters sent to New York City’s mayor and his group that advocates for gun control contained traces of the deadly poison ricin, police have said.

Thousands of people joined a rally for gun control in the US capital Washington, just days before the Senate begins debating new measures proposed by President Barack Obama.

New York mayor Michael Bloomberg has long been a persistent advocate for tighter gun laws, but in the wake of the Newtown school massacre, his voice has become one of the most recognisable in the United States.

A man fatally shot a former colleague near the Empire State Building and then was killed by police in a gun battle that wounded nine bystanders outside one of New York’s most popular landmarks.