A Muslim-American lobby group is endorsing Democratic presidential nominee Hillary Clinton and vowing to "work to mobilise Muslim Americans across the United States to defeat Donald Trump this November".

Emerge USA declared its support for Clinton on Tuesday, six weeks before the November 8 general elections.

The group describes itself as the largest Muslim-American political action committee at the national level with a mission of "politically empowering" Muslims in the United States.

During the campaign, Trump, a New York real estate tycoon, has proposed both a ban on Muslims entering the United States, advocated monitoring of mosques and criticised the parents of a Muslim US Army officer who died fighting in Iraq.

Empower USA blasted Trump's "bigoted" immigration plans.

"These proposals are anti-American at their core, and violate the very essence of what our founders foresaw for this great nation," the group said.

"This election is profoundly important to Muslim Americans, our civil rights - our very existence."

The endorsement followed Monday night's first of three debates between Clinton and Trump, the nominee of the conservative Republican Party.

"In this election cycle, the safety and security of our community has never been in more danger, and during last night's debate, the choice of who would fight to defend our freedoms was made abundantly clear. Donald Trump directly threatens the values, principles and civil rights of Muslim Americans living here and abroad."

Clinton issued a statement thanking Emerge USA for its endorsement and said that Trump "continues to paint a dark and misleading picture of the Muslim-American community".