Hundreds of thousands of protesters have taken to the streets in hundreds of US cities for the second Women's March, marking the first anniversary of President Donald Trump's inauguration.

Key points: March organisers say Donald Trump's policies propel women into activism

March organisers say Donald Trump's policies propel women into activism Women rallied in 250 US cities including Washington, New York, LA and Chicago

Women rallied in 250 US cities including Washington, New York, LA and Chicago Women encourage more people to vote to take 'power to the polls'

The coordinated rallies in Washington, New York, Los Angeles, Chicago and about 250 other cities were a reprise of the mass protests that marked the beginning of Mr Trump's presidency.

Los Angeles Mayor Eric Garcetti estimated the crowd in the city at 600,000, and said it was largest march in the country.

Organisers have hailed it as a new era of political activism, saying they are hoping to keep up the momentum from last year's anti-Trump march, which became one of the biggest mass protests in US history.

"While we have this President celebrating his one-year anniversary, let's give him an 'F' for his performance," Democrat Nancy Pelosi told a Washington crowd, adding: "We don't agonise, we organise."

Despite the protests, Mr Trump said the day was a good opportunity for women to celebrate his time in office.

"Get out there now to celebrate the historic milestones and unprecedented economic success and wealth creation that has taken place over the last 12 months," he tweeted.

"Lowest female unemployment in 18 years."

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Joblessness among women was 3.7 per cent in December, below the overall US unemployment rate of 4.1 per cent, according to the Labor Department.

But Katie O'Connor, a 39-year-old lawyer from Knoxville, Tennessee, who travelled to the National Mall, said she wanted Mr Trump out.

"I don't believe this administration is going to do anything good for women," she said.

Many of the protesters wore pink knit "pussy hats", which were created for last year's march as a reference to a comment made by Mr Trump about female genitalia.

The caps quickly became a symbol of women's empowerment and opposition to the new President in the early days of his administration.

"We want to continue the fight to resist this President and the policies we're against," said Sara Piper, 59, a geologist from Reston, Virginia.

US President Donald Trump has been criticised for rolling back birth control and equal pay protections. ( Reuters: Eduardo Munoz )

Organisers hope to build on the energy felt by Mr Trump's opponents after his surprise election victory in 2016 and channel it into gains for progressive candidates in November's midterm elections, using the theme "Power to the Polls".

They want to register a million new voters and get more strong advocates for women's rights into office.

Clinton praises marches, calls for 'same power in the voting booth'

Activists said Mr Trump's policies rolling back birth control and equal pay protections pushed many women to protest.

"We will make our message heard at the polls this fall," Emily Patton, a rally organiser, told thousands of demonstrators at the Reflecting Pool on Washington's National Mall.

"That is why we are urging people to register to vote today."

Mr Trump's election campaign rival Hillary Clinton said the marches were "a testament to the power and resilience of women everywhere", and she wanted to see "that same power in the voting booth this year".

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The rallies come during what has been seen as a pivotal year for women's rights, with the #MeToo and #TimesUp social media efforts against sexual harassment and abuse that were born out of a string of scandals in Hollywood, Washington and elsewhere.

The Washington rally featured Democratic politicians from neighbouring Virginia, including Senator Tim Kaine, who blamed Mr Trump and Republicans for the shutdown of the government on Saturday.

"The Trump shutdown is due to the inability of the Republican Party to do basic governing, like making a budget," he said to cheers.

Some critics said this year's march lacked a focus, and libertarian think tank and senior analyst at the Reason Foundation Shikha Dalmia said targeting an issue such as immigration would have had a greater impact.

"Beating the feminist drum just seems to me beside the point. Maybe they are trying to cast as wide a net as possible," Ms Dalmia said.

A White House spokesman did not respond to a request for comment on the marches.

The marches will be followed by more events on Sunday, including in Las Vegas, a key battleground state in the 2018 midterm congressional elections.

The voter registration campaign will target swing states held by Republicans, such as Nevada, and districts considered a toss-up ahead of November's midterm elections.

Women's March organisers urge people to register to vote in a bid to get more women elected. ( AP: Craig Ruttle )

Reuters