Millions of people lost relatives, homes and livelihoods in the earthquake
A month on from the Sichuan earthquake, Chinese officials have imposed tight security in some of the damaged areas, apparently to prevent protests.
Police in the city of Dujianyan stopped parents from holding a memorial ceremony at the rubble of a collapsed school where their children died.
Journalists were told they were banned from the city, and some were detained.
Parents have been demanding to know whether poorly-built schools played a part in the deaths of their children.
Thousands of schoolchildren were among the 87,000 people killed or missing after the massive 12 May earthquake.
Five million people also lost their homes, and officials estimate rebuilding work will take at least three years.
Looking for blame
The government's rapid response to the disaster has drawn widespread praise, but tensions are emerging as efforts shift to focus on reconstruction.
Parents who lost their children want to know why so many schools collapsed - something many blame on shoddy construction linked to local corruption.
Early on Thursday, a number of parents tried to get the remains of the Dujiangyan primary school to hold a small memorial, but a line of police stopped them from going inside.
The Chinese media had been instructed not to cover this kind of story, and a member of the BBC and five other journalists were detained for a short time for approaching the parents.
"This is not censorship," one policeman told the BBC's China correspondent James Reynolds.
Police also reportedly cordoned off a collapsed middle school in Juyuan after a 50-strong crowd gathered outside.
"All we want to do is remember them this day," Zhao Deqin, a mother whose 15-year-old twin daughters died in the disaster, told Reuters news agency.
In Beichuan, where about 1,300 children were killed, parents were able to gather around the remains of a school to grieve.
Mu Qibing, whose 17-year-old son was killed, told Reuters: "They said this building was strong and quake-proof, but when we saw it, the concrete was like talcum powder and the steel was as thin as noodles."
"None of us have seen our children yet, not even after one month."
Beichuan suffered such severe damage that the whole town will be rebuilt in a new location.
'What happens now?'
China is not holding any formal commemorations to mark the one month anniversary of the earthquake.
The government has ordered its departments to cut spending so that funds can be allocated to reconstruction efforts.
"The government has done a good job so far but we need to know what is going to happen to us," Bai Tao, who lost his home and business, told AFP news agency.
"We business people have real problems. But all we've gotten is free water and instant noodles. We need to know about the future," he said.
The BBC's China analyst, Shirong Chen, says the earthquake has brought about unexpected political and social change that will directly affect the reconstruction effort.
The unprecedented openness in China's media coverage is likely to continue, he says, and there will be a demand for accountability in the way tens of millions of dollars of donations are used.
The earthquake has also injected a strong sense of national unity, he adds, with volunteers from all over the country pouring into disaster areas to work alongside soldiers and rescue teams.