The national flag in Tiananmen Square in central Beijing flew at half mast after a ceremony at dawn, Xinhua state news agency reported early on Monday.
State television said the Olympic torch relay through China would also be suspended for three days.
On the eve of the official mourning period, a fresh tremor in southwestern China killed three people, injured 1,000 others and sent thousands fleeing their homes into the streets.
The tremor, one of the strongest aftershocks since the May 12 earthquake, hit Jiangyou city in Sichuan, Xinhua said.
It was 5.7 in magnitude and brought down a large number of houses, damaged 377 km (235 miles) of roads and six bridges, rescue authorities said late on Sunday.
In the provincial capital, Chengdu, some 200 km (125 miles) south of the epicenter, thousands fled swaying buildings, Xinhua said.
The official death toll stands at nearly 32,500 from the original quake of 7.9 magnitude that rattled Sichuan province.
Some 220,000 people are reported injured and a further 9,500 are thought to be still buried under the rubble in Sichuan. Most are feared dead. Officials have tried to keep people from the area because of aftershocks and a build-up of water in blocked rivers.
Xinhua said the most dangerous mass of water was only about 3 km upstream from Beichuan town where rescue workers saved a man on Sunday from under the remains of a hospital.
China says it expects the final death toll to exceed 50,000. About 4.8 million people have lost their homes.
Survivor's Legs Amputated
Late on Sunday, a woman was also pulled out of the rubble in Yingxiu after a 56-hour rescue operation during which her legs were amputated, Xinhua reported. A man was earlier found alive in a collapsed office building in Maoxian county, it said.
The military moved to quell concerns over the safety of its nuclear facilities, including the main nuclear weapons research laboratory, close to the affected zone.
"I could say in a responsible manner that all these facilities are safe and secure," Ma Jian, a senior People's Liberation Army officer, told a news conference in Beijing. "There is no problem at all."
Offers of help have flooded in and rescue teams with sniffer dogs and specialized equipment from Japan, Russia, Taiwan, South Korea, the United States and Singapore are assisting. Donations from home and abroad have topped 6 billion yuan ($858 million).
Statistics from past earthquakes show some victims have survived up to nearly a fortnight under rubble.
Yet Fujiya Koji, head of the Japanese rescue team in Sichuan, conceded: "Generally by this stage the likelihood of survival is low. They say they have been finding some in Beichuan and we'll certainly keep trying."
In Beichuan, hard hit by the quake and which many people fled on Saturday following warnings a dam may collapse, worried relatives quarreled with police who tried to prevent them entering the area, citing safety reasons.
"I've traveled all this way, and I don't know where my father is," said Chen Shiquan, who had come back from work in the neighboring province of Qinghai to look for his father. "To let me get this far and then not let me in is too cruel."