A top lawyer and senior adviser to Monaco's Prince Albert II was brought before a judge on Saturday amid a lengthy probe by investigators into a long-running scandal involving corruption and influence-peddling.

Lawyer Thierry Lacoste, a former school friend and adviser to Albert, was taken into police custody on Wednesday as officials investigate a massive property project that fell through, the Monaco Public Prosecutor's Office said.

Lacoste was brought before the investigating judge after being questioned and two extensions of his detention. The judge opened formal criminal proceedings against him on suspicion of forming a criminal organization, actively bribing a national public official, passively exerting undue influence and money laundering. He has been placed under judicial supervision, the office said.

Lacoste denies the allegations and is still presumed innocent.

He told the police that the accusations had been made by a property magnate "who is dissatisfied because he was not awarded certain property projects in the Principality of Monaco," his solicitor's office said in a statement.

The magnate is attempting to exploit the principality's justice system for his own benefit, the statement said.

Those who had fought against corruption for years and whom it had not been possible to silence were now being accused of "corruption or exerting undue influence."