Upon returning to South Vietnam, Đán was involved in negotiations with Diệm, but did not join the government, and then became the centre of open opposition to Diệm, starting the Democratic Opposition Bloc and the "Thời Luận" newspaper, which stridently criticised the government. Despite ransackings by a mob of regime supporters, the newspaper's closure by a government court, and his blacklisting from university employment, Đán continued his opposition activities and was elected to the National Assembly in 1959, but was prevented from taking his seat. He then joined the paratroopers' coup of 1960 as a spokesperson after it started and was then jailed in a labour camp when Diệm loyalists crushed the revolt. Đán was released in 1963 upon Diệm's overthrow and assassination, and went on to become Foreign Minister and Deputy Prime Minister before escaping Vietnam during the Fall of Saigon.