Trading has been suspended at the New York Stock Exchange over technical issues.
Denying that the stoppage was due to cyberattack, the NYSE said it "chose to suspend trading on NYSE to avoid problems arising from our technical issue".
The New York Stock Exchange has suspended trading in all stocks, for unknown reasons.
A NYSE spokeswoman was not immediately available for comment, though some reports are blaming earlier technical issues.
US government officials have told NBC News that there is no indication that the shutdown is not related to cyberattack.
Twitter shares fell 4.6% on the day after going on sale for the first time. The stocks traded at 70% above their IPO price, avoiding the flop experienced by its social media rival Facebook last May.
Facebook shares slumped dramatically in the first few weeks after their stock exchange debut.
Shares in Twitter went on sale Thursday and almost doubled in value from its opening price offer of $26 (£16.26).
Many tweeters have noticed that their posts are helping the social networking site increase in value, so TIME magazine has created a calculator that works out how much Twitter 'owes' you.
Share prices in the company have rocketed above the $50 mark meaning Twitter's value is over $24 billion (nearly £15bn).
ITV News has tweeted 60,599 times which, according to the calculator, means $279,128 (£173,651) is 'owed' to us.
However, popstar Justin Bieber - who was recently overtaken by Katy Perry in Twitter followers - is 'owed' a staggering $20,923,841 (over £13m) by the company.
Shares in Twitter went on sale today and almost doubled in value from its opening price offer of $26. The social networking site has never turned a profit, despite being worth $19 billion tonight.
Trading on the New York Stock Exchange under the symbol "TWTR", shares opened at $45.10 (£28.11), 73% above their IPO.
ITV News' Business Editor Laura Kuenssberg reports on Twitter's initial success:
Twitter Inc has closed at $44.90 per share on the first day of trading on the New York stock market, showing an increase of 73% on the day, but below the opening price of $45.10.
Twitter can help generate an additional £16.06 ($25.62) per tweet in revenue for businesses, according to online marketing data company SumAll, as the social media site made its successful debut on the New York stock market today.
Here are some key figures from the analysis:
- A single business tweet on average generates an additional £16.06 ($25.62) in revenue, compared to not tweeting at all
- Overall, successfully integrating Twitter into digital marketing efforts increases total revenue by 1-2%
- Comparatively, however, it found that Instagram followers are worth 10 times more than on Twitter
Consolidated volume on Twitter shares have hit five million after only 30 minutes of trading, according to Reuters, as the social media company launched on the New York stock market.
Shares for the popular micro-blogging site soared as much as 92% but have stabilised around the $45 mark.
Twitter has made a "stunning" debut on the New York Stock Exchange, said a ETX Capital market strategist, as shares opened at $45.10. It meant that just seven years after starting up, the micro-blogging site was worth $31 billion. Ishaq Siddiqi added: