WASHINGTON (AFP) - A key stage of President Barack Obama's health care reform law hit a stumbling block on Tuesday when millions of people flocked to browse online insurance but were frustrated by technical glitches.

The launch of healthcare.gov went ahead despite a US government shutdown incited by Republicans opposed to the law, and Mr Obama vowed that six in 10 people could find insurance there for less than US$100 (S$125) per month.

But many of the 2.8 million users the government reported having counted for the day were blocked from entering the site due to heavy traffic.

"With any new product launch, there are going to be glitches as things unfold," said Marilyn Tavenner, administrator of the Centers for Medicare and Medicaid Services.

"We certainly had a high volume of consumers creating accounts this morning," she told reporters. "We added capacity, and made some adjustments to the system to handle that."

She declined to specify how many people had signed up on the first day of what is to be a six month process, saying only: "We can confirm that people have enrolled."

Known widely as "Obamacare," the Affordable Care Act was passed by Congress and signed by Obama in 2010, with a promise to help over 30 million uninsured people get health care coverage.

Despite its ambitions to revolutionise American health care, the plan is not free, and will not be able to cover tens of millions of people in America who lack full insurance.

Advocates have touted it as a major step toward universal health care, while opponents like Republican House speaker John Boehner have insisted the law "is not ready for prime-time."

Indeed, for much of Tuesday, the new health insurance exchanges that were to begin enrolling people via the website healthcare.gov were jammed.

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

An attempt by an AFP reporter to check prices offered in Virginia resulted in the message: "Important: Your account couldnt be created at this time. The system is unavailable."

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.

"Today is launch day for Obamacare and so far it's like a giant online traffic jam on the first day of summer vacation," said independent technology analyst Jeff Kagan.

"No one knew what to expect, but chaos is what we have." Mr Obama said the main website had been slowed because more than one million people visited before 7am (7pm Singapore 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," Mr Obama said outside the White House, flanked by a handful of citizens who would benefit from the reform.

"We are going to be speeding things up in the next few hours to handle this demand that exceeds anything we expected." About seven million Americans are expected to seek coverage by 2014, according to the Congressional Budget Office.

The policies available on the health exchanges range in price according to a person's income, location, family size and the level of coverage desired on a scale of bronze, silver, gold or platinum.

All Americans must sign up for some kind of health insurance by Jan 1 or face a fine.

The government said there would be an average of 53 plans to choose from in the federally-facilitated marketplace.

Most Americans have health insurance - 82.6 per cent according a July report from the Department of Commerce based on data from the 2010 Census.

About half get it through their employer, and about 30 per cent are enrolled in government programs like Medicare, mainly for seniors, and Medicaid, which covers the poor and disabled.

That leaves 17.4 per cent of Americans with no health insurance whatsoever, or about 53 million people.

An analysis in the journal Health Affairs earlier this year projected that around 30 million Americans will still not get health insurance through the Affordable Care Act.

Those gaps are due in part to the Supreme Court's decision to allow some states to opt out of expanding Medicaid programs, coupled with a cut in funding to safety-net hospitals.

"The ACA, whatever its merits, will fall well short of its stated goal of providing affordable care for all Americans."