BEIRUT, Lebanon (AP) _ Police said Israeli helicopter gunships attacked Shiite Moslem guerrilla strongholds in south Lebanon today after a roadside bomb blast killed four pro-Israel militiamen, but the Jewish state denied it had launched an air raid.

The police report said Israeli rocketing and strafing attacks covered the Litani River basin about seven miles north of the border. Information on casualties was not immediately available, it said.

In Tel Aviv, an official with the military command who spoke on condition of anonymity said, ''There was no air raid, none at all.''

The police report said a one-hour raid, the second in two days, started at 10 a.m. and covered a 5 1/2 -mile stretch of the Litani basin between the bridges of Khardali and Qaqaieh near the inland market town of Nabatiyeh.

The raiding gunships fired ''several phosphorous rockets that started fires along both ranks of the Litani and heavily strafed the whole region,'' the report said.

The reported attack came five hours after four militiamen of the Israeli- backed South Lebanon Army were killed and six others injured by a roadside bomb that exploded under their armored personnel carrier near Markaba, about one mile north of the border with Israel.

Israeli military sources in Tel Aviv said two of the wounded militiamen were taken by helicopter to an Israeli hospital.

Six SLA members were killed Friday in an attack by Shiite Moslem guerrillas of the Hezbollah, or Party of God, in Israel's self-designated ''security zone.'' In apparent retaliation for that attack, Israeli helicopter gunships on Sunday blasted Hezbollah strongholds.

At least three people were killed and eight wounded in Sunday's Israeli raid on the villages of Khirbat Selim and Kabrikha, in the Ghanaian-policed zone of the U.N. peacekeeping force in south Lebanon.

Pro-Iranian Shiite zealots recently have intensified their attacks against the 1,500-strong South Lebanon Army, which is backed by about 1,000 Israeli troops in the six- to 10-mile-wide security zone.

The enclave was carved out in 1985, when Israel withdrew most of its occupation army from Lebanon, in a effort to create a buffer against cross- border attacks on northern Israel.

More than 100 South Lebanon Army militiamen have been killed since the Israeli pullout in June 1985.