In the third day of intensified shelling, a powerful car bomb explosion devastated a narrow street in east Beirut today, killing at least seven persons.

During the morning rush hour, a red Renault E rigged with dynamite and mortars rocked the crowded alley near a school in the Christian Sin Fil district. The blast trapped motorists in flaming cars and shoppers under smoking debris. More than 80 people were reported wounded. Two buildings were destroyed and at least 11 shops were damaged, civil defense sources said.

In Paris, a television station reported receiving its first communication from its four television crew members who have been held hostage in Lebanon since March, The Associated Press reported.

The increased violence in Beirut preempted a planned trip by Lebanon's newly appointed Christian Maronite patriarch to the Moslem section of the city where he was to meet with the Sunni mufti of the republic.

Today's car bomb followed two days of fierce shelling that spread to residential areas in east Beirut and the densely populated Shiite suburbs. About 50 persons have died and 120 have been wounded in the bombings.

The heavy shelling also follows a gesture by President Amin Gemayel for a rapprochement with neighboring Syria, and it dampened a campaign by Christian politicians to gain support for a new plan for national reconciliation, which has been rejected by Moslem leaders.

Judicial sources said four suspects were detained for questioning today, including an Armenian who runs a printing press, two Egyptians and a Lebanese. However, sources with the Lebanese Forces, a Christian militia, said two persons were arrested for suspected involvement in today's bombing. The sources said that the way in which the car was rigged and detonated pointed to Syrian intelligence service tactics.

Today's was the sixth car bomb in the Christian-controlled enclave since January and the second in exactly the same spot in one year. One victim, 22-year-old Sophie Rizk, who lost an eye in an explosion last May 22, had one of her legs blown off today.

The Associated Press reported the following from Paris:

Four members of a French television crew held hostage in Lebanon since early March have sent letters and photographs indicating for the first time that they are well, their television station announced today.

Color snapshots show the four men, unshaven, reading a May 14 edition of the Lebanese newspaper L'Orient le Jour, said Paul Nahon, deputy director of the Antenne 2 station. The station said the letters were being given to the families. They would not reveal their contents.

It is the first communication from the French captives. Government officials said the letters and photographs were given to the television station last night, but refused to say how they were obtained.

Pierre-Henri Arnstam, editor-in-chief at Antenne 2, said: "It's a good sign. We feel good."

The four Frenchmen were seized March 8 in Moslem-controlled west Beirut as they returned from covering a meeting of Hezbollah, an extremist Shiite Moslem group allied with Iran. Several days later a group calling itself the Organization of Revolutionary Justice asserted responsibility.

France has been trying recently to improve relations with Iran, which were strained by the French sale of sophisticated military equipment to Iraq, which is at war with Iran. Deputy Prime Minister Ali Reza Moayeri, the highest-ranking Iranian to visit Paris since the 1979 revolution, said here yesterday that talks with French officials had been "positive on the whole."

French authorities also said that Prime Minister Jacques Chirac spoke by telephone with President Hafez Assad of Syria late yesterday or early today.