The opposition Socialists called the halting of Nepszabadsag's print and online operations was a "black day for the press" and called a demonstration outside the paper's offices at 16:00 GMT.
Owners Mediaworks said however that the suspension of the loss-making publication was for business reasons and until "the formulation and realisation of a new concept".
Nepszabadsag is Hungary's biggest-selling broadsheet newspaper and has frequently been critical of Orban, most recently before last weekend's referendum on refugees.
Orban has often been accused of turning public media into a government mouthpiece while large swathes of the private media sector have been bought by government-friendly oligarchs, critics say.
On Saturday there was speculation that Nepszabadsag might also be sold, but Mediaworks, the Hungarian unit of an Austrian media magnate, made no mention of any sale.
A senior Nepszabadsag editor said that journalists, who had prepared stories for a Monday edition, were suddenly prevented from entering the workplace, and have received letters informing them of their suspension.
"We are in shock, it's like a coup. Of course they will try and paint this as a business decision but it's not the truth," the journalist, who did not wish to be named, said.
"It's a huge blow to investigative journalism and freedom of the press. Nepszabadsag was the largest group of quality journalists in Hungary trying to defend basic freedoms, democracy, freedom of speech, and tolerance."
Mediaworks, which bought Nepszabadsag and several other Hungarian titles in 2014, said that its circulation had tumbled by 74 percent in the last 10 years, racking up losses of around 5 billion forints (16.4m euros, $18.4m).
The title must try to find a business model appropriate to market trends, the company said in a statement on local news agency MTI, and called on "all affected by the move to concentrate on that task".
Before Hungary's referendum last Sunday on a EU plan to relocate migrants around the bloc, the paper, one of Hungary's oldest, published several stories about scandals involving politicians close to Orban.
Source: AFP
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