In the disaster-hit Tohoku region and in cities across Japan, people attended memorial services and concerts, got together with family and close friends and watched special reports on television to remember the triple disasters that changed so many lives and triggered national soul-searching.

A nationwide minute of silence was observed at 2:46 p.m., exactly one year after the magnitude-9.0 tremor slammed Tohoku and set off the area’s worst tsunami in centuries. Roughly 19,000 people died or went missing—mostly victims of the giant waves that washed away scores of communities along the coast. Some trains stopped running to allow people a moment to pray and reflect. Flags flew at half-staff.

It was also a day to witness newly emerged activism in a nation known for political indifference. Antinuclear demonstrations took place in major cities, including a protest in front of the headquarters of Tokyo Electric Power Co., the operator of the stricken Fukushima Daiichi nuclear plant. The main demonstration in Tokyo drew some 30,000 protesters, according to the organizers, making it one of the largest antinuclear demonstrations in the past year.

“It is important to pray for the victims of the March 11 disaster. But prayers alone won’t change the country,” said Miyako Maekita, the organizer of the event in Tokyo.

In Fukushima, a rally held in a baseball stadium was attended by 16,000 people, according to the organizers, including many farmers and fishermen whose livelihoods have been lost since the multiple meltdowns.

In the coastal city of Rikuzentakata in the northeast, families of the deceased, many wearing black, laid flowers on flattened plots of land where their loved ones are believed to have died a year ago. More than 2,000 people, or about 10% of the population, died in the 15-meter waves that swallowed the city in Iwate Prefecture.

In the city’s Takata neighborhood, an area close to the waterfront where many people died, about 100 people gathered for a Buddhist ceremony Sunday morning. The mourners chanted along with a Buddhist monk whose voice cracked as he wept during the ceremony.

A large makeshift tent was set up for a ceremony outside a local elementary school in the city, near a punctured high school devastated by the tsunami. Dressed in black, many people arrived as early as two hours before the start.

“My chest was full of grief and I couldn’t help but cry,” said Takeko Kano, a 67-year-old grandmother who came to the ceremony hoping she would feel a sense of peace over her missing daughter.

“You know, they still haven’t found my daughter…but there is some sense of closure,” she said.

After a full year, the disaster’s impact continues to reverberate throughout the nation. In Tohoku, nearly 350,000 people remain displaced from their homes, with many living in cramped temporary housing, some jobless, some without hope, as they face an uncertain future.

The Fukushima accident severely contaminated surrounding towns within and beyond a 20-kilometer exclusion zone, forcing tens of thousands of residents to relocate. The government is set to review the various evacuation zones at the end of March, and declare some areas uninhabitable for decades.

While the accident has generated huge safety concerns about Japan’s nuclear industry, by forcing the closure of other nuclear reactors, it has also sharply reduced Japan’s energy-supply capabilities. That has sent households and corporations alike into extreme energy-saving mode, a trend that could weigh on the nation’s economy for years to come.

Tokyo is spending ¥20.9 trillion ($250 billion)—around the size of the annual gross domestic product of Portugal—to rebuild the northeast. The spending further bloats Japan’s government debts, already the worst among major nations, fueling concerns about their sustainability.

Still, for most Japanese, Sunday was a day for remembering those who died and sober reflection on what has been lost.

In Tokyo, some 1,200 people, including top government officials, attended a ceremony at the National Theatre of Japan. The guest of honor was Emperor Akihito, who made a truncated but highly symbolic appearance just three weeks after having heart-bypass surgery.

Speaking slowly but in a clear and strong voice, the emperor devoted most of his speech to thanking people for their sacrifices and hard work. “I can never forget there were firefighters and others who lost their lives because they rescued others and ensured safety, even as they knew their own lives were in danger,” said the 78-year-old emperor, facing the altar on the stage adorned with white chrysanthemums shipped in from Tohoku.

Standing next to him was Empress Michiko, wearing a black kimono, the most formal attire for the Japanese people. “I would like to express my deep gratitude to people who toiled for the victims and for the disaster-hit areas; to people who have worked to cope with the nuclear accident.”

Leading proceedings was Prime Minister Yoshihiko Noda, who spent the day in Tokyo. “This is not only a day of mourning, but a day to renew our resolve to rebuild,” he said. Mr. Noda, who was finance minister at the time of the disaster, was promoted to the premiership in September after his predecessor, Naoto Kan, lost popular support amid criticism over the government’s handling of disaster relief and the nuclear accident.

Among the tens of thousands of protesters who gathered in Tokyo, some headed to the parliament building in Nagatacho to form a human chain around it in the early evening. Some held candles, shouting out their opposition to the restarting of nuclear reactors that were shut down after the Fukushima accident. “Give back Fukushima!” they shouted. “Protect our children’s future!”