An Indian billionaire close to Prime Minister Narendra Modi is trying to buy a broadcaster seen as the last major critical voice on television, stoking fears about media freedom in the world's largest democracy.

Gautam Adani -- Asia's richest person, with interests ranging from Australian coal mines to India's busiest ports -- announced late Tuesday that his firm had indirectly acquired a 29-percent stake in NDTV and was bidding for a further 26 percent.

NDTV said that the move came "without any discussion" with the broadcaster, "or the consent of the NDTV founders", journalist Radhika Roy and economist Prannoy Roy.

Its two channels, one in Hindi and one in English, stand out among India's myriad rolling news broadcasters for inviting on critics of the government as well as their hard-hitting reporting.

It has already been hit by a slew of legal cases that its owners said were a result of its reporting.

On Wednesday morning an employee at NDTV told AFP that there was a "general sense of shock and disbelief" in the newsroom following the announcement.

"We only found out from other news agency flashes and channels about the takeover and then all hell broke loose," the employee said, asking to remain anonymous.

"People are still trying to figure out what has happened and what will happen. There is a sense of uncertainty since it is only a matter of time till the new management comes in."

Geeta Seshu, founder of the Free Speech Collective, an independent organisation that advocates press freedom, said that "the space for independent journalism has shrunk alarmingly over the last few years."

"The few brave journalists who continue to put out information are also battling court cases, are lodged in jail for long periods without bail, attacked or silenced permanently," Seshu told AFP.

Seshu said that while NDTV has been "struggling" commercially for some time, "the manner of this takeover is shocking, given the naked display of economic and political muscle".

The Adani group's closeness to the government was "hardly a secret", she added.

Self-made billionaire Adani, 60, this year overtook fellow Indian Mukesh Ambani to become Asia's richest man, with a net worth of $139 billion according to Forbes, behind Jeff Bezos and ahead of Bill Gates.

Modi and Adani both come from the western state of Gujarat, and the latter's conglomerate has expanded aggressively in recent years, including into new areas like airports and renewable energy.

But this growth into capital-intensive businesses has raised alarms, with Fitch Group's CreditSights warning on Tuesday that the group was "deeply overleveraged".

Ambani's wealth and influence have also grown under Modi -- he now owns more than 70 media outlets that are followed by at least 800 million Indians, according to Reporters Without Borders.

This includes a majority stake in Network18, one of the biggest media conglomerates in the country, which owns several leading broadcasters.

NDTV's integrity has been a bright spot in a media landscape compromised by increasing corporate control, said P Sainath, the founder and editor of the grassroots reporting network People's Archive of Rural India.

"Under the circumstances and under the pressure they've worked in, they really stand out," he said.

Hartosh Singh Bal, journalist at Caravan magazine -- a rare critical voice among print media -- said the takeover could bring the curtain down on "the only channel left that could be called partly independent".

"The government influence on the media is growing. The control of what I call oligarchs -- the Adanis and the Ambanis -- is also growing and it will keep on growing," he told AFP.

"This (takeover) means there is almost no independent media left and that shrinking space is extremely dangerous."