Beijing beefed up the city's subway security on March 30, while Shanghai's preparations for the World Expo meant thousands of police were already safeguarding the underground rail system, according to China's senior public security officers.

Fu Zhenghua, Director of the Beijing Public Security Bureau, confirmed that security in the capital city had been increased after the double suicide bombing in Moscow's subway system on March 29.

"The number of police patrols accompanied by sniffer dogs was increased at subway stations, where police would make more frequent inspections and cross-examine suspicious passengers," he said.

Song Youguo, Director of the Operation Center of the Rail Branch under the Shanghai Public Security Bureau, told Xinhua that Shanghai had a very tight security regime with 10,000 police monitoring the city's subway system, as well as 400 security staff, in preparation for the World Expo starting in May.