Japan fell silent in prayer on Sunday, six months after an earthquake and tsunami left 20,000 dead or missing and sparked a nuclear crisis on its Pacific coast. At 2:46 pm the eerie wail of warning sirens rang out, marking exactly six months since the the 9.0-magnitude quake struck offshore, unleashing towering waves which swallowed ships, sea walls, vehicles and whole communities. In towns and villages along the devastated coast, mourners gathered to remember the dead, while anti-nuclear rallies were held in Tokyo and other cities over the Fukushima crisis -- the worst nuclear accident since the 1986 Chernobyl disaster. In the town of Minamisanriku, where 900 people were killed and 60 percent of the buildings were destroyed, about 2,000 people dressed in black gathered at a public gymnasium to observe a moment's silence. "We never give up hope and vow to unite as one in building a new town so that we can make up for the sacrifice of precious lives of many people," Minamisanriku mayor Jin Sato said during the remembrance service. The disaster crippled cooling systems and sparked meltdowns at the Fukushima Daiichi plant, some 220 kilometres (138 miles) northeast of Tokyo, forcing tens of thousands to evacuate, who still have no idea when they can return home. "The most difficult thing is that I have lost my job and it is hard to work out plans for my children," Takahiro Murakami, 35, told AFP in Minamisanriku. "The biggest shock was to see my town disappear." In the major fishing port of Ishinomaki, where 4,000 people lost their lives in the tsunami, people gathered on a hill overlooking the town below, where mounds of debris and wrecked vehicles litter the waterfront. As the sirens sounded out they folded their hands and prayed, many of them in tears. A 600-kilometre (375-mile) stretch of Japan's scenic northeast coast was left devastated by the disaster, and 4,100 people are yet to be accounted for, while 23 million tonnes of debris still need to be disposed of. Meanwhile, more than 70 anti-nuclear rallies were held across Japan, according to media reports. In Tokyo about 1,000 people, including many families, marched through the trendy streets of Shibuya as children held placards reading: "No Nukes." About 1,300 people marched past the head office of the plant's operator, Tokyo Electric Power Co., and formed a human chain around the economy ministry, which is responsible for nuclear power. Several thousand demonstrated in another hub of Shinjuku, beating drums and chanting: "We don't need nuclear power plants" and "Save the children". The six-month anniversary came amid embarrassment for Prime Minister Yoshihiko Noda's new government after trade minister Yoshio Hachiro resigned on Saturday over remarks deemed insensitive to Fukushima evacuees. After touring the Fukushima plant and the no-go zone with Noda on Thursday, Hachiro sparked fierce criticism by referring to the neighbourhood around the plant as a "town of death". Noda held a meeting with his ministers on post-disaster reconstruction on Sunday and apologised for Hachiro's gaffe after observing a minute's silence. "Our struggle with the Fukushima accident is only half done," Noda said. "Without solving the accident, Japan cannot regain (international) trust." The prime minister on Saturday travelled to ravaged Miyagi and Iwate prefectures for the first time since taking office last month, when he replaced Naoto Kan, who resigned amid criticism over his handling of the disaster. The government was accused of underplaying the full scale of the nuclear crisis, and allowing political infighting to overshadow recovery efforts. Rebuilding the muddy wastelands of the northeastern "Tohoku" region is expected to cost hundreds of billions of dollars and take up to a decade. Areas close to the Fukushima Daiichi nuclear plant may be uninhabitable for even longer. Radiation fears became a feature of daily life after cases of contaminated water, beef, vegetables, tea and seafood. But the government has been at pains to stress the lack of an "immediate" health risk.