Families of the passengers on the missing Malaysia Airlines flight MH370 say that six months of agonising uncertainty have been made harder still by the company's attitude, and by detention and even beatings by Chinese police.

Speaking before Monday's six-month anniversary of the flight's disappearance on 8 March, relatives of the 154 Chinese passengers returned again and again to the question that has baffled everyone: in an age when individuals can be tracked to a street by their smartphones, how can a huge Boeing-777 simply vanish en route from Kuala Lumpur to Beijing, and fly on for hours, without anyone noticing? The practical answers cannot address the sense of sheer implausibility.

A search that once drew in 26 countries is now focusing on 60,000 sq km of the southern Indian Ocean seabed off the west coast of Australia, at a cost of tens of millions of pounds.

A report in June by the Australian Transport Safety Bureau said the most likely scenario was that the flight coasted into the ocean on autopilot, with all 239 people on board, while the pilots were unresponsive because of hypoxia, often caused by depressurisation.

That assessment was reached purely to help locate the aircraft and, even if correct, sheds no light on the plane's unexplained deviation from its route – turning back across the Malaysian peninsula and then heading south. Investigators believe it was deliberate: communications systems appear to have been disabled just as it left the country's airspace, moments before its sudden diversion.

Malaysia's initially muddled response to the disappearance, and the lack of answers half a year on, have fuelled suspicion among the victims' families. While some accept that MH370 crashed into the ocean and suggest the early contradictions and confusions were not surprising in a developing country, others fear a conspiracy and are insistent that the search teams are looking in the wrong place.

"I used to be able to believe people," said Dai Shuqin, whose sister was on board. "But now I've turned to prayer and fortune-telling."

Two-thirds of the passengers were Chinese and some families – though they stress they are angry with Malaysia and the airline, not their own government – say their pain has been compounded by official pressure.

One woman, who asked to be called only Kelly, said she and another relative were detained after protesting at the Malaysia Airlines office. When other family members rushed to the station where she was being held, they were beaten.

"I was strangled by a tall officer and almost got choked. I tried to escape from him but about six or seven policemen came up and beat me. My daughter wanted to take a picture for evidence, but a policewoman took her mobile phone away and pushed her to the ground, dragging her hair. She too was forced into the interrogation room. I felt sad and desperate," wrote one of those relatives.

According to other family members in a subsequent case, 14 adults and two small children were taken to the police station after they decided to spend the night at the Malaysia Airlines office. Police had earlier ordered them to leave, saying it was against regulations to stay there despite the permission of the company.

Another relative said almost 30 people had been taken to police stations at one time or another, and that hotels refused to accept their bookings for meetings.

"We have felt helpless and alone, as if we were abandoned by the government," he said.

Beijing police said they would look into allegations of beatings and detentions.

Families say they want all data available about the flight, the precise methodology used in defining the search area, and other evidence such as footage of passengers boarding the plane. They complain that Malaysia Airlines and Malaysia have brushed such requests aside, giving them conflicting information and failing to respond at all to some queries.

There is particular anger at the decision to set up the Malaysian Airlines family support office in a distant suburb of Beijing – a long walk from the nearest bus and subway lines.

"It feels as if the deaths of our relatives are just numbers to them," said Jiang Hui, another relative.

Malaysia Airlines said in a statement that it "has deeply invested and continues to invest in supporting the physical and emotional needs" of the families, saying it has been staying in touch with around 1,000 relatives through press statements, phone calls and emails.

"We are sparing no expense or effort to take care of them and that also means providing them with timely and accurate information."

Experts say air disaster investigators rarely release much information prior to concluding their reports, and note that search teams have provided extensive reports.

While the area under exploration is still vast, it is much smaller than the original 600,000 sq km search area. A detailed survey of the ocean floor in a zone far off the western coast of Australia has revealed features such as undersea volcanoes and steep cliffs.

Warren Truss, Australia's deputy prime minister, said last month that the next, more intensive phase of the hunt would cost around A$52m (£30m) if it lasted 12 months, but that the searchers hoped they would find the plane sooner.

Even if the plane were found, it is far from clear what information it could yield after so long under such pressure.

Meanwhile, the Malaysian-led international investigation continues to search for any possible mechanical problems – which might have led pilots to divert the plane – and potential "human factors" among the crew and especially the pilots. They have described these as hijack, sabotage, and personal or psychological problems. Months on, there is no sign of a breakthrough.

"The first thing is to find the plane; the second is to find the truth," said Jack Song, another relative.

"Our goal isn't just to fight for ourselves. It's for the safety of thousands and thousands of passengers flying in future. If you can't solve this problem or give us the truth about what happened, how can you make sure such things will never happen again?" he asked.

Earlier this year, the Malaysian prime minister, Najib Razak, acknowledged the government had made mistakes and said the country had already tightened security.

But he went on to call for a string of other measures, including an examination of whether pilots should be able to turn off communication systems; the extension of cockpit voice-recording times so they cover the entire flight and not just the last two hours; and extending the life of the electronic beacons on black boxes from 30 days to 90, as the EU has suggested, to make them easier to find.

That appears to be one area in which most family members are in agreement with the Malaysian government. They called for similar technical improvements and measures to address other issues highlighted by MH370's disappearance – such as the fact that most countries do not routinely check passengers' documents against Interpol's stolen passports database.

Sarah Bajc, whose boyfriend, Philip Wood, was on the flight, pointed to the sums airlines spend on frills such as inflight entertainment systems as proof that the industry could afford to make safety improvements if it chose.

"What's important?" asked Song. "Profits or lives?"