(Adds context on bond, background)
CARACAS, Oct 29 (Reuters) - Allies of Venezuelan opposition leader Juan Guaido on Tuesday filed a U.S. lawsuit requesting that state oil company PDVSA's 2020 bond be declared null, according to a filing in the U.S. District Court for the Southern District of New York.
The bond, backed by shares in U.S. refiner Citgo Petroleum Corp, has gone into default after a Monday deadline passed without the cash-strapped company making a $913 million payment. Citgo is majority-owned by Petroleos de Venezuela, S.A., known as PDVSA.
The U.S. Treasury Department, which maintains a broad sanctions program against Venezuelan President Nicolas Maduro, last week temporarily blocked transfers or sales of Citgo shares linked to the bond.
"Plaintiffs seek a judgment declaring that certain notes issued by PDVSA ... are invalid, illegal, null and void ab initio, and thus unenforceable," reads the filing, which also seeks an injunction blocking investors from taking the collateral.
The lawsuit named as defendants MUFG Union Bank, the bond's trustee, and GLAS Americas LLC, the collateral agent. It identifies the plaintiffs as PDVSA and two of its subsidiaries.
Venezuela's information ministry and PDVSA, with an ad hoc board named by Guaido to oversee foreign holdings, did not immediately respond to requests for comment.
Guaido has been recognized by more than 50 countries as Venezuela's legitimate president, but Maduro continues to control the day-to-day operations of government and the Venezuelan operations of PDVSA.
Maduro lost control of Citgo when Washington recognized Guaido.
Reporting by Brian Ellsworth Editing by Leslie Adler and Richard Chang