BEIJING — Officials in critical Communist Party and government posts in Chongqing who are considered loyalists of Bo Xilai, the city’s deposed party chief, are being detained as part of the wide-ranging investigation into Mr. Bo and his family, according to a Chongqing official and other people with knowledge of political appointments in the city.

The detentions are part of an attempt by the central Communist Party to dismantle Mr. Bo’s support network and build a case against him and his wife, Gu Kailai, who is under investigation in the killing of a British businessman, Neil Heywood. Mr. Bo is being investigated for “serious disciplinary violations.”

The detentions and, in some cases, replacements of Mr. Bo’s allies began soon after party leaders ousted him on March 15 as the Chongqing party chief, said people in Chongqing and Beijing, who spoke on the condition of anonymity because of the delicacy of the situation. Some analysts say the purging of Mr. Bo has presented the party’s top echelon with its biggest challenge since the Tiananmen Square protests and massacre in 1989.

The downfall of Mr. Bo, who comes from a prominent revolutionary family but acquired many enemies in his political career, began when his former police chief in Chongqing, Wang Lijun, showed up at the United States Consulate in Chengdu on Feb. 6 with what he said was evidence tying Mr. Bo’s family to Mr. Heywood’s mysterious death in Chongqing last November.