They included the president of Rome's transport company and his two immediate predecessors, two senior officials from the company which runs the Rome underground railway and regional transport system, a former railways official and a judge whose task was to ensure that public works contracts were awarded honestly.
The managing secretary of the Rome Christian Democrats, who is also a senator in the national parliament, was told he was under investigation.
The seven - Christian Democrats, Socialists and a former Communist - were the latest batch of political dignitaries, officials and company representatives to be rounded up as part of the Milan investigations into political corruption.
Investigations into political corruption are also in progress in Venice, Reggio Calabria, Rome and other cities.
The Milan magistrates were acting on allegations that bribes totalling pounds 16m were paid over a period of 10 years by a north Italian engineering firm in exchange for contracts to buy buses.