Former New York Mayor Rudolph Giuliani has officially joined the running for the Republican nomination for the 2008 US presidential election.
Mr Giuliani supports gun control and abortion rights
The Federal Election Commission said Mr Giuliani had filed a "statement of candidacy" - a one-page form outlining a candidate's wish to seek office.
Mr Giuliani was widely praised for his response to the 9/11 attacks on the World Trade Center in New York in 2001.
He has remained in the public eye and is known as a moderate Republican.
The so-called statement of candidacy filed on Monday puts Mr Giuliani on the same level, legally, as Republican candidates John McCain and Mitt Romney.
Unlike his main rivals, Mr Giuliani has been ambiguous about whether he would ultimately put himself forward for the Republican nomination.
Mr Giuliani set up an exploratory committee in November last year and said that he was "testing the waters".
Under US law, setting up such a committee allowed Mr Giuliani to travel the country to gauge support for a candidacy without formally declaring himself as a candidate.
Correspondents say being mayor, even in a city as big and important as New York, is not commonly regarded as normal preparation for a presidential campaign.
But in the chaos which followed the devastating attacks on the World Trade Center, Mr Giuliani emerged as a defiant and unifying leader, earning him the honorary, but unofficial, title of "America's mayor".
Since then, Mr Giuliani has remained in the public eye and is popular with the American people, particularly Republicans.
However, his relatively moderate views may make it difficult for him to persuade mainstream Republicans that he should be their presidential candidate.
His support for same-sex civil unions and embryonic stem cell research would put him to the left of most of his party members.