A large crowd gathers at Cafe Du Monde in the French Quarter on March 15, 2020 in New Orleans, Louisiana. The state government has placed a ban on large public gatherings and postponed the April 4 primary until June 20 due to the coronavirus (COVID-19).

"Large events and mass gatherings can contribute to the spread of COVID-19 in the United States via travelers who attend these events and introduce the virus to new communities," the CDC said. It cited conferences, festivals, parades, concerts, sporting events and weddings as examples of gatherings that should be postponed.

The CDC said individuals and organizations should reschedule large events and gatherings of any size should be reconsidered unless organizers can protect vulnerable people, ensure proper hand hygiene and social distancing.

The Centers for Disease Control and Prevention is urging people across the U.S. to cancel or postpone events with 50 or more attendees for the next eight weeks to try to contain the fast-moving coronavirus pandemic, the agency said in revised guidance issued Sunday.

The COVID-19 outbreak has quickly infected nearly every state in the U.S. in just weeks, spreading from roughly 100 people on Mar. 1 to almost 3,300 people by Sunday, according to data compiled by the CDC, World Health Organization and Johns Hopkins University. The virus has already killed 62 people in the U.S. and the number of infections and deaths here will continue to rise, public officials say. Tens of thousands more cases are suspected in the U.S., but haven't been diagnosed yet, because of delays and constraints on testing, state and local leaders have complained.

Last week, New York Gov. Andrew Cuomo said the state could see a similar spread of COVID-19 just like in China, South Korea and Italy, where the new coronavirus has millions of people under lockdown and has shuttered commerce.

"What makes you think that the virus in China, the virus in South Korea, the virus in Italy wasn't going to react any differently than the virus here?" he said. "You are going to see the same trajectory that you saw in China, South Korea and Italy, and it is going to happen here as the virus spreads because of the way it is actually contagious."

COVID-19 was first reported in Wuhan, China less than three months ago. As it is a new virus, humans don't have any natural immunity to it, scientists say. Across the world, it's spread to more than 162,000 people, killing more than 6,000.

The number of cases outside of China, the epicenter of the outbreak, increased thirteenfold "and the number of affected countries has tripled," WHO Director-General Dr. Tedros Adhanom Ghebreyesus said just before declaring a global pandemic on Wednesday. He said the number of cases and deaths will rise in the coming weeks, and he scolded some world leaders for failing to act quickly enough or drastically enough to contain the spread.

"We're deeply concerned both by the alarming levels of spread and severity, and by the alarming levels of inaction," he said. "We have rung the alarm bell loud and clear."

U.S. President Donald Trump tried to calm the nation on Sunday, telling Americans to "relax" and not hoard food.

"There's a very contagious virus, it's incredible, but it's something that we have tremendous control of," Trump said at a White House press conference minutes after the Federal Reserve announced new steps to shield the U.S. economy from the effects of the pandemic.

The Federal Reserve cut interest rates to essentially zero on Sunday and launched a massive $700 billion quantitative easing program to shelter the economy from the effects of the virus. Despite the aggressive move, the market's initial response was negative. Dow futures pointed to a decline of some 1,000 points at the Wall Street open Monday morning.

The Trump administration is weighing "all options" to curb the outbreak in the U.S., including an outright halt to domestic air travel — a step not taken since the Sept. 11, 2001 attacks.

"We continue to look at all options and all options remain on the table," Chad Wolf, acting secretary of the Department of Homeland Security, said at the White House press briefing when asked if a domestic travel ban was possible.

Read the CDC's new guidance below:

Large events and mass gatherings can contribute to the spread of COVID-19 in the United States via travelers who attend these events and introduce the virus to new communities. Examples of large events and mass gatherings include conferences, festivals, parades, concerts, sporting events, weddings, and other types of assemblies. These events can be planned not only by organizations and communities but also by individuals. Therefore, CDC, in accordance with its guidance for large events and mass gatherings, recommends that for the next 8 weeks, organizers (whether groups or individuals) cancel or postpone in-person events that consist of 50 people or more throughout the United States. Events of any size should only be continued if they can be carried out with adherence to guidelines for protecting vulnerable populations, hand hygiene, and social distancing. When feasible, organizers could modify events to be virtual. This recommendation does not apply to the day to day operation of organizations such as schools, institutes of higher learning, or businesses. This recommendation is made in an attempt to reduce introduction of the virus into new communities and to slow the spread of infection in communities already affected by the virus. This recommendation is not intended to supersede the advice of local public health officials.

CNBC's Leslie Josephs, Tucker Higgins, Christina Wilkie and Steve Liesman contributed to this article.