(Chayan Majumdar)Behrampore: Bengal chief minister Mamata Banerjee on Monday launched a scathing attack on the BJP over its headquarters in Delhi, saying it should be "ashamed of" "world's biggest party office" and questioning the source of funds for the construction. BJP chief Amit Shah had described the building as the biggest office of any party in the world. "They are boasting about inaugurating the world's largest party office, when they should be ashamed of it. "Our party office has been built with our own money. At the meeting, Mamata went on to bring up the fraud at the Punjab National Bank and added other banks to the mix.