A spokesman for state-owned Bangladesh Railway said the guards had been deployed to prevent saboteurs from removing "tracks and fishplates" on main inter-city routes on the eighth day of a transport blockade.
"Six trains have been derailed since the opposition blockade began," spokesman Syed Zahrul Islam told AFP, adding that more than a dozen people had been injured in the derailments.
"As a result, we have no option but to deploy 8,328 (guards) across the network to prevent further acts of sabotage."
BNP leader Khaleda Zia, who boycotted a general election last year over fears it would be fixed, is currently confined to her office as Prime Minister Sheikh Hasina tries to prevent her from spearheading the protests.
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Zia's spokesman said the BNP leader had no intention of softening her demands in order to secure her release from the office, where she has effectively been a prisoner since January 3.
"She won't halt protests until the government agrees to hold election under a neutral administration," Shimul Biswas told AFP by phone from inside Zia's leadership compound in Dhaka's upmarket Gulshan district.
Biswas said at least 2,000 BNP supporters and officials have been detained in the last two weeks. On Monday police arrested five senior officials including its publicity secretary Habibur Rahman from outside a private television station where he was a guest on a talk show.