He denies all wrongdoing and says he is being persecuted by left-wing prosecutors. He says the alleged sex parties were elegant dinners where the female guests performed "burlesque" shows. El Mahroug denies having sex with Berlusconi.
Prosecutors say Berlusconi should serve one year in jail for paying for sex with a minor and should be given five years' imprisonment and a life ban from holding public office for the abuse of office charge, which they consider more serious.
In May 2010 the then prime minister called a Milan police station to instruct officials to release El Mahroug, who was being held on suspicion of stealing a bracelet.
A Brazilian prostitute who lived with El Mahroug had called the premier on his mobile phone to tell him she had been arrested.
Berlusconi's lawyers say he made the call to avoid a diplomatic incident because he believed that El Mahroug, who is actually Moroccan, was the grand-daughter of former Egyptian President Hosni Mubarak. The prosecution says he was anxious to cover up the relations he had with her at his sex parties.
The media tycoon has recently used his own television stations to promote his version of events, with his flagship Canale 5 channel broadcasting a prime-time documentary on the so-called "Ruby Trial".
Whatever the verdict on Monday, Berlusconi will not go to jail before he has exhausted his right to two appeals under the Italian legal system, which could take years.
The political consequences, however, could come sooner. Letta, from the centre-left Democratic Party (PD), is likely to be rooting for a not-guilty verdict in the interests of his fledgling government's stability.
The case is only part of Berlusconi's legal problems. Last month an appeals court upheld a four-year jail sentence against him for orchestrating a tax fraud scheme connected with his business dealings - leaving him just one more appeal, at the supreme court, which could come within a year.
Despite Berlusconi's professions of loyalty to Letta, many analysts believe he will eventually prefer to gamble on fresh elections, in which he could potentially become prime minister once again, rather than risk a definitive sentence.
Even if Berlusconi opts to keep backing the government, a guilty verdict would make parts of the PD highly uneasy and increase the coalition's instability, said Giovanni Orsina, professor of contemporary history at Rome's Luiss University.
"The PD would be in the same majority with a person who has been condemned in the first degree for juvenile prostitution which is not a light issue," he said. "It would add up to a difficult situation."