United States president Barack Obama has blamed the government shutdown on an "ideological crusade" by Republicans to derail his healthcare program.

More than 800,000 government workers will spend a second day on unpaid leave after the shutdown came into effect at midnight on Monday (local time), when Congress failed to meet a deadline to bring in a new budget.

Despite this, the centrepiece of Obamacare - subsidised health insurance - is now being sold online across all US states, in a move Mr Obama says will allow tens of millions of uninsured Americans to get medical coverage.

Mr Obama says it is strange that Republicans have stopped the government without achieving their goal of stopping his reforms.

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"This Republican shutdown did not have to happen - I want every American to understand why it did happen," he said in the White House Rose Garden.

"This Republican shutdown threatens our economy when millions of Americans are still looking for work and businesses are getting some traction," he said.

"So the timing is not good."

Mr Obama is blaming the Republican's right-wing Tea Party movement for the shutdown.

Sorry, this video has expired Newt Gingrich tells Republicans to stick to their guns

"They've shut down the government over an ideological crusade to deny affordable health insurance to millions of Americans," he said.

"One faction of one party in one house of Congress in one branch of government shut down major parts of the government because they didn't like the law."

But former Republican House speaker Newt Gingrich told the ABC's Lisa Millar that Republicans should stick to their guns.

"Obama wants them to bow. I don't think it's a very American habit to bow," he said.

"The president sooner or later has to recognise he's part of the constitutional process.

"He's not a king. He's not above us. We don't bow to him.

"The day he's willing to negotiate, they'll find an agreement."

Obamacare's insurance shops overwhelmed by traffic

Meanwhile, technical glitches hampered the first day of Obamacare's online shopfront, with the health insurance exchanges hit by heavy traffic as they went live.

What is Obamacare? Affordable Care act was passed by Congress in 2010

Affordable Care act was passed by Congress in 2010 Promises to help tens of millions of uninsured Americans get health coverage

Promises to help tens of millions of uninsured Americans get health coverage Under the plan, people can buy cheap insurance on healthcare.gov

Under the plan, people can buy cheap insurance on healthcare.gov Most coverage will cost less than $US100 per month

Most coverage will cost less than $US100 per month Policies vary according to person's income, location, family size and level of coverage desired

Policies vary according to person's income, location, family size and level of coverage desired All Americans must sign up to some form of health insurance by January 1 or face a fine

All Americans must sign up to some form of health insurance by January 1 or face a fine 82.6 per cent of Americans already have health insurance - half get it through employer

82.6 per cent of Americans already have health insurance - half get it through employer 17.4 per cent (about 53 million people) are uninsured

17.4 per cent (about 53 million people) are uninsured Studies project about 30 million of uninsured people still won't get insurance despite Obamacare

Frustrated users flocked to Twitter to complain that healthcare.gov and other sites linked to the exchanges were not working.

The NY State of Health site said that "overwhelming interest" had led to two million visits in the first two hours of the launch, and asked people to come back later if they had been unable to log in.

Mr Obama said the website would speed up later in the day, and it had been slowed because more than one million people visited before 7:00am (local time) alone.

"There were five times more users in the marketplace this morning than have ever been on Medicare.gov at one time. That gives you a sense of how important this is," he said.

"We are going to be speeding things up in the next few hours to handle this demand that exceeds anything we expected."

Under the scheme, Mr Obama said most people should be able to get coverage for less than $US100 per month.

About seven million Americans are expected to seek coverage by 2014, according to the Congressional Budget Office.

The Daily News takes a dim view of the US government shutdown. ( Daily News/ABC )

Under the first government shutdown in 17 years, monuments are closed and 800,000 public servants have been told to stay at home.

The effects of the budget battle ranged from the poignant to the symbolic.

A social program that provides food to poor women and children was hit and the Statue of Liberty was closed to visitors.

Under the Capitol, where rival clans of lawmakers failed to find an eleventh-hour deal to pass a federal budget, the National Mall was sealed off by a sign marked "area closed."

Prospects of a swift resolution to the crisis were unclear and economists warned that the struggling US economic recovery could suffer if the shutdown drags on for more than just a few days.

Sorry, this video has expired US government shutdown forces closure of Washington memorials ( Washington correspondent Lisa Millar )

Unknown consequences for US economy

While Mr Obama's supporters blamed the deadlock on a relatively small faction of Tea Party conservatives, Congress as a whole has lost prestige.

"House of Turds," headlined the New York Daily News above a parody picture of Republican House Speaker John Boehner as the corrupt anti-hero of Washington television drama House of Cards.

Who is affected? 800,000 government employees sent home without pay

800,000 government employees sent home without pay 1 million asked to work without pay

1 million asked to work without pay Department of Education staff will be severely hit

Department of Education staff will be severely hit Skeleton staff to remain at Department of Energy to oversee nuclear arsenal

Skeleton staff to remain at Department of Energy to oversee nuclear arsenal National parks, Statue of Liberty and Alcatraz will close

National parks, Statue of Liberty and Alcatraz will close Washington's Smithsonian Institution, including 19 galleries and the National Zoo will close

Washington's Smithsonian Institution, including 19 galleries and the National Zoo will close Passport offices in the US will shut down and visa applications will go unprocessed overseas

Passport offices in the US will shut down and visa applications will go unprocessed overseas In Washington, libraries will be closed, parking tickets won't be issued

In Washington, libraries will be closed, parking tickets won't be issued Read more about the different federal departments affected here.

"DC cess-pols shut down government. They get paid while nation suffers," it trumpeted.

A few hours into the shutdown, Republicans in the House appointed delegates, or conferees, to negotiate with the Senate later on a spending plan to get the government up and running again.

But if they still want to tinker with Obamacare, the Senate will not negotiate, an aide to Senate Majority Leader Harry Reid said.

Mr Obama warned that a government shutdown could badly damage an economy which has endured a sluggish recovery from the worst recession in decades.

"We may not know the full impact of this Republican shutdown for some time," he said.

"But we do know a couple of things. We know that the last time Republicans shut down the government in 1996, it hurt our economy."

The president also warned Republicans against using a crucial mid-October deadline to raise the government's $US16.7 trillion debt ceiling as leverage to try to reverse the health care law or achieve other political objectives.

"Congress, generally, has to stop governing by crisis," he said.

"I'm not going allow anybody to drag the good name of the United States of America just to refight a settled election or extract ideological demands."

A debt default that would result if Congress fails to raise the debt ceiling when it is reached in less than three weeks could be devastating, Mr Obama said.

The threat of default in 2011 resulted in a painful debt rating downgrade, he added.

"If they go through with it this time, and force the United States to default for the first time in its history, it would be far more dangerous than a government shutdown, as bad as a shutdown is. It would be an economic shutdown," he said.

A rickshaw driver sleeps near the US Capitol in Washington DC after the government went into shutdown mode. ( AFP: Jewel Samand )

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