Malaysia took a "significant" step backwards in human rights in 2013, a year that appeared to mark the end of Prime Minister Datuk Seri Najib Razak's promises of reform, Human Rights Watch said Tuesday.

The US-headquartered organisation's annual report said Malaysia's rights situation deteriorated sharply after Najib's now 57-year-old ruling coalition was stung by a historic setback in elections in May.

"The election was followed by a significant deterioration in human rights and the apparent abrupt end to Prime Minister Najib's oft-touted reform agenda," the report said.

"Relevant developments in the second half of 2013 included passage of new and revised laws again permitting administrative detention without trial, new arrests of opposition activists for organising peaceful protests, and repression of political speech."

Facing ebbing voter support, Najib had in 2011 abolished some repressive laws and pledged to protect rights.

He acknowledged growing pressure for reform in the multi-ethnic country that has been ruled by the same ethnic Malay Muslim-dominated regime since independence.

This included scrapping the draconian Internal Security Act (ISA), which had allowed virtually indefinite detention without trial, and promises to relax curbs on the media.

The ISA had long been criticised as a tool to silence government critics.

However, the government in October approved new anti-crime measures restoring detention without trial despite a public outcry.

Traditional media in the Muslim-majority country also remain firmly under the government's thumb.

Authorities in December suspended a business magazine after it published an article alleging huge sums were being spent on foreign travel for Najib and other expenses.

The opposition has branded Najib's earlier reform pledge as a cynical ploy to secure votes ahead of last May's polls.

The coalition dominated by the United Malays National Organisation (UMNO) lost the popular vote for the first time ever but clung to power thanks to what critics call widespread parliamentary gerrymandering.

"Malaysia in 2013 was marked by a 'tale of two Najibs' -- promising legal reforms before the election and restoring repressive laws after it," Phil Robertson, the rights group's deputy Asia director, said in a statement.

Since the election, the weakened premier is widely believed to be under pressure from powerful UMNO conservatives who want to turn the clock back on reform.

Human Rights Watch also said the government "continued to bring dubious criminal charges against its political opponents," including prominent activists who face trial for violating the Sedition Act.

Najib had previously promised to abolish the Sedition Act.

His office did not immediately respond to a request for comment.