Sri Lanka’s new government has moved swiftly to dismantle the authoritarian rule of the outgoing president, Mahinda Rajapaksa, pledging to stop blocking websites, end intimidation of the media and reopen an investigation into the murder of a high-profile journalist.

Analysts had described the election as the most significant in Sri Lanka for decades and a last chance for democracy in the island nation. Maithripala Sirisena, the 63-year-old veteran politician who left the previous government to lead the opposition to a surprise win, was formally sworn in on Friday night.

Many had predicted widespread violence, particularly as the opposition campaign gathered momentum. In the event, the poll went smoothly with a record turnout exceeding 81%. The final count gave Sirisena 51.2% of the votes. Rajapaksa, who called early elections confident of winning an unprecedented third term, conceded defeat early.

Opposition figures claimed on Saturday night that the smooth transition was due to senior officials and the army refusing orders from the outgoing government to deploy troops in Colombo, the cultural and commercial capital.

Speaking on Saturday at a press conference alongside an army spokesman, Rajitha Senaratne, a parliamentarian who defected to Sirisena and is likely to be an important minister in the new cabinet, said a plan to declare a state of emergency had been thwarted.

“The army chief was under pressure to deploy, but he ... declined to carry out the instructions. Rajapaksa tried to stay in power till the last moment. It was when he realised that he could not, that he left,” Senaratne said.

The MP said the new government would reopen an investigation into the murder of Lasantha Wickrematunge, editor of the Sunday Leader, who was gunned down in Colombo in 2009 and was a trenchant critic of Rajapaksa.

Sirisena’s new prime minister, Ranil Wickremesinghe, told reporters on Friday that they are “free to report whatever you want without the fear of being abducted”, and said a programme of internet censorship enforced by the former regime was also being lifted.

Lal Wickrematunge, Lasantha’s brother, told the Observer on Saturday that he could provide new information about the killing to any inquiry. “There is no better signal of the new government’s avowed intention to assure media freedom than opening investigations into the assassinations, abductions and assaults of journalists, media personnel and media institutions,” he said.

Activists critical of the government have been routinely intimidated in recent years, with several disappearing after being picked up by white vans believed to be used by the security services. They include Prageeth Eknaligoda, a political cartoonist, who was last seen being bundled into a vehicle near his office in January 2010.

Legal documents filed by the Sri Lankan government during habeas corpus hearings in Colombo describe Eknaligoda as having been “abducted by an organised group”. Last year his wife called for international pressure on the government to either release her husband or inform her of his fate.

Rajapaksa won a second term easily in 2010, surfing a wave of popularity after overseeing a final bloody victory over ethnic Tamil separatists that ended a crippling 26-year civil war.

His decision to seek early polls may have been more a recognition of growing unpopularity than a show of strength, however. The benefits of brisk economic growth have failed to reach the poor, especially in rural areas, and there was anger at corruption and apparent nepotism.

An adamant refusal to act on reconciliation with the Tamil minority and growing sectarian violence denied Rajapaksa votes among other constituencies, while votes from the Tamil-dominated former war zone in the country’s north and in areas with large Muslim communities appear to have played a key role in Sirisena’s victory.

Rajapaksa fell out with the west over war crime allegations involving the deaths of thousands of Tamil civilians in the final phases of the civil war in 2009, and refused to cooperate with a UN-mandated investigation, leading to Sri Lanka becoming increasingly close to China.

Moves to concentrate power on the president and weaken the judiciary led the then UN commissioner on human rights, Navi Pillay, to warn of authoritarianism in late 2013.

Activists welcomed the announcements on Saturday. “If the government is serious about establishing democratic values in the country, then strengthening media freedom and freedom of expression is an integral part of that,” said Sunil Jayasekara, convenor of the Free Media Movement, a campaigning coalition in Sri Lanka.

“But words would not be enough – there needs to be decisive action that shows the government is serious.”

Many newspapers scrambled to keep up with the sudden shift in power. The state-run Daily News, which ran a fierce campaign against the new president, relegated Rajapaksa to a stamp-size photo at the bottom of the front page. Just a day earlier, the newspaper had printed that the idea of defeat for the incumbent “seems to be a figment of the imaginations of various interested parties”.

Rajpal Abeynayake, the editor since 2011, explained that as it was state-owned the newspaper’s job was to “reflect the line of whatever government is in power”.

“If the government changes, so does the newspaper. It’s as simple as that. If they want to change that practice they could, but so far no government has done it,” he said.