A Malaysian government crackdown under its Sedition Act is creating a climate of fear in the country, according to rising numbers of critics who say it could stunt a recent flowering in freedom of speech.

About 40 people -- mostly opposition politicians including leader Anwar Ibrahim, but also student activists, lawyers, academics and a journalist -- have been investigated, charged or convicted under the act this year, activists say.

The crackdown, accelerating in recent weeks, is widely seen as an attempt by Malaysia's longtime regime to reverse years of increasingly boisterous speech that has coincided with tremendous electoral gains by the opposition.

"It has a chilling effect," said Ibrahim Suffian, head of independent pollster Merdeka Centre, who adds that many Malaysians are beginning to "self-censor".

"I think we haven't seen the worst of things."

The act outlaws speech deemed to incite unrest or insult Muslim-majority Malaysia's largely ceremonial Islamic royalty. It can bring three years in jail.

International organisations have condemned the crackdown, including a group of United Nations human rights experts who said last week it "threatens freedom of expression by criminalising dissent".

The US embassy in Malaysia joined in on Friday, saying it had "raised our concerns about the rule of law and human rights with the Malaysian government".

It urged the government "to apply the rule of law fairly, transparently, and apolitically".

- Selective prosecution -

Malaysia has seen years of increasingly open, largely Internet-enabled public criticism of the ruling United Malays National Organisation (UMNO), which is frequently tainted by corruption, rights abuses and other scandals.

UMNO tightly tethers traditional media, but its critics have harnessed the power of the unshackled Internet in a country with huge rates of social media use.

Outspoken independent news sites have helped inspire millions to envision a political alternative in a country where power is monopolised by the Malay Muslim majority, to the chagrin of its sizeable religious minorities.

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But many fear that flowering is now under threat.

"(The sedition crackdown) has definitely had a negative impact on freedom of speech in Malaysia," said Andrew Khoo of the Malaysian Bar Council, and "perpetuates a cycle of ignorance and intolerance".

Khoo helped organise a Kuala Lumpur protest march Thursday by hundreds of lawyers who decried the Sedition Act as "an antithesis of democracy, rule of law, justice and human rights".

"It's a huge issue," said Irin Tan, 22, a law graduate who took part in the march.

"It's an abuse of human rights. Now we even have to be careful what we put on Facebook."

Some of the sedition charges are questionable at best, critics say.

An opposition state lawmaker was charged in August for uttering "Damn, damn UMNO" to colleagues.

Meanwhile, UMNO-aligned groups are allowed to make incendiary racial statements. One influential conservative Malay nationalist recently called for the burning of Bibles.

Power tussles within UMNO may be behind the clampdown.

Prime Minister Najib Razak, who took the helm of one of the world's longest ruling regimes in 2009, promised two years ago to scrap the Sedition Act to placate demands for openness.

But he has abandoned political reform under pressure from UMNO conservatives, especially after he stunningly lost the popular vote in elections last year, though he clung to control of parliament.

In August, authoritarian former prime minister Mahathir Mohamad, who remains an influential conservative, harshly criticised Najib's reform rhetoric, saying it would lead to instability.

Shortly after, the pace of sedition arrests picked up sharply.

Najib's administration has since been unclear on whether the Sedition Act will indeed be replaced. His office declined comment.

- 'Losing control' -

Analysts said Najib -- who portrays himself aboard as a moderate reformer -- may be allowing the crackdown so he can look tough ahead of UMNO's annual assembly in November, or to tighten the reigns ahead of looming political flashpoints.

These include the introduction next April of a controversial goods and service tax expected to lead to unpopular price rises.

Opposition leader Anwar also faces an October 28-29 hearing that will produce a final ruling in a controversial sodomy conviction and five-year jail sentence, which could potentially spark opposition protests if he is imprisoned.

Anwar calls the episode politically motivated.

Yap Swee Seng, a prominent activist and spokesman for a coalition of more than 100 NGOs opposing the Sedition Act, said Najib may simply have "lost control" to conservatives.

Since the law shields the country's Islamic Malay monarchs, concern is growing that UMNO could also be seeking to use Malaysia's royalty as a bulwark against democracy, as several recent sedition cases involve comments allegedly insulting to the monarchy.

University lecturer Azmi Sharom has been charged for allegedly questioning the actions of a state sultan in a 2009 political controversy, and has since launched a constitutional challenge to the Sedition Act.