Bomb was hidden in box of fruit and detonated amid morning traffic.
Action aimed at sending message to Pyongyang and Beijing.
A bomb has targeted the convoy route of Pervez Musharraf within minutes of him passing, police say.
Relations between the two countries continue to suffer after the arrest of a junior Indian diplomat.
Ex-military ruler pleaded not guilty and has always claimed that the charges are politically motivated.
Japanese premier Shinzo Abe compared China's actions in the disputed East and South China Seas to Russia's offensive in the Crimean peninsula.
Supporters of Prime Minister Yingluck Shinawatra had been demonstrating at the building.
North Korea made an insulting attack on President Park Geun-hye following her comments on its nuclear program.
Polling is being held in 91 seats in 14 states, including in the capital Delhi and the key state of Uttar Pradesh.
The 2009 presidential election was similarly tainted by allegations of election fraud.
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