As temperatures soar to around 40C this weekend, thousands of Romans will flock to nearby beaches to roast in the sun, play on the slot machines and dance in sticky seaside nightclubs. They will not be the only ones feeling the heat.

On Friday in an operation that prosecutors said revealed the extent of organised crime in the coastal suburb of Ostia near the Italian capital, 51 people were arrested on suspicion of mafia-related activity.

The crackdown, which involved about 500 police officers as well as dog support units, patrol boats and a helicopter, was described as one of the biggest anti-mafia sweeps ever carried out in the Rome area.

Its aim was to hit at the heart of gangs that prosecutors say have been carving up the coastal territory and sharing its considerable spoils for the past 20 years.

Located about 15 miles south-west of the capital near Leonardo da Vinci airport, Ostia's sandy beaches prove a popular weekend destination for city-dwellers seeking to escape Rome's stifling heat.

Particularly targeted in the operation on Friday were members of three clans – the Fasciani, Triassi, and D'Agati – whom investigators suspect of carrying out criminal activities including drug trafficking and extortion.

The Triassi are believed to have close ties to the Sicilian mafia. The website of Il Fatto Quotidiano, a daily newspaper, headlined the raids: "Welcome to Cosa Nostra beach".

The alleged infiltration by criminal networks in Ostia's political administration emerged this month when police raided the town hall's permit office and placed an employee and local contractors under investigation on suspicion of rigging bids for beach contracts in favour of another mafia clan, the Spada.

The move prompted Rome's new mayor, Ignazio Marino, to announce that permits to manage Ostia's coastline would henceforth be handled directly from the capital. He said his administration would fight to curb "the underworld infiltration" of Ostia.

"In recent years, the Roman coastline has become fertile ground for criminal activities, the scene of bloody clashes between clans and criminal gangs who seek to control significant parts of the city's economy," he said.

One of the most startling incidents in the increasingly bloody turfwar in Ostia came in November 2011 when two criminals, Giovanni Galleoni and Francesco Antonini, were shot dead in the town centre in broad daylight.

In a separate but equally dramatic anti-mafia operation on Friday morning, police in the southern region of Calabria made dozens of arrests in the city of Lamezia Terme, about 40 miles south of Cosenza, some of which concerned a suspected car-crash scam in which payouts were allegedly used to provide criminals with drugs and arms.

Police said the raids had targeted a panoply of local people suspected of involvement in the scheme, ranging from insurers and lawyers to car repairers. There were also arrests of suspected hitmen on suspicion of several killings between 2005 and 2011, police said.

A Calabria senator in the former prime minister Silvio Berlusconi's centre-right Freedom People (PdL) party was being investigated for suspected vote-buying but had not been arrested, they added.

Guido Marino, police chief in nearby Catanzaro, said the raids revealed a flourishing criminal system in the city that had drawn in not only fully paid-up members of criminal gangs but "professionals above suspicion".

"This was a mafia system which not only bloodied Lamezia Terme with murders but which also bled dry a part of [the city's] already fragile economy," he was quoted as telling the Ansa news agency.

Calabria, one of Italy's poorest regions, is the home to the 'Ndrangheta, now Italy's most formidable organised crime syndicate, which has grown far beyond its southern origins into a hugely powerful force thought to control much of the cocaine trade in Europe.