Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.

Passage: Warsaw's first stock exchange was established in 1817 and continued trading until World War II. It was re-established in April 1991, following the end of the post-war communist control of the country and the reintroduction of a free-market economy. Today, the Warsaw Stock Exchange (WSE) is, according to many indicators, the largest market in the region, with 374 companies listed and total capitalization of 162 584 mln EUR as of 31 August 2009. From 1991 until 2000, the stock exchange was, ironically, located in the building previously used as the headquarters of the Polish United Workers' Party (PZPR). Question: Whose former headquarters was the WSE located in until 2000?
Polish United Workers' Party