A powerful explosion ripped through a Shiite Moslem religious and community center in Maarakeh in southern Lebanon today, killing 12 persons, including two key leaders of the Lebanese resistance to Israel's continuing occupation of the area, and wounding at least 40.

The incident, one of the bloodiest in southern Lebanon since the beginning of the Israeli occupation in 1982, came less than 30 hours after the last units of a large Israeli force left Maarakeh following a search of the village. Shiite leaders charged that those Israeli forces had planted the bomb that exploded today, but the Israeli Army, in a statement issued in Tel Aviv, denied involvement.

Less than an hour after the explosion, Israeli troops raided the main hospital in Tyre, the largest nearby city, according to hospital officials and a Reuter reporter on the scene. They fired weapons, beat up the hospital director, seized at least eight people lined up to give blood for victims of the bomb and smashed doors in an apparent search for residents of Maarakeh, the eyewitnesses said.

In Tel Aviv, an Israeli military source said troops detained a number of people in Tyre after a demonstration in the hospital courtyard, Reuter reported.

The violence in Maarakeh and Tyre came a day after the Israeli Cabinet voted approval of the second stage of a three-stage withdrawal of Israeli forces from Lebanon.

Since the completion of the first stage on Feb. 16, which brought the Israeli line south to the Litani River, near Tyre and Maarakeh, Shiite forces have stepped up their attacks on Israeli occupation forces.

Israel has responded with a crackdown that included raids on suspected Shiite guerrilla strongholds such as Maarakeh. Israeli military officials said Saturday that they found large amounts of heavy weapons and ammunition in Maarakeh.

Israeli Prime Minister Shimon Peres and Defense Minister Yitzhak Rabin, in separate statements today, both defended the Israeli crackdown in southern Lebanon, Washington Post correspondent Edward Walsh reported from Jerusalem.

The explosion in Maarakeh killed Mohammed Saad, 25, the chief coordinator of the guerrilla actions against the Israeli forces, and Khalil Jeradi, commander for the Tyre area, according to Shiite and other sources.

Nabih Berri, leader of the Shiite Amal militia and a Lebanese Cabinet minister, announced the deaths, calling Saad and Jeradi "martyrs," and urged that the Lebanese Army be strengthened and "given the order to confront the occupying enemy."

Lebanese President Amin Gemayel summoned Stephen Lyne, the number two official at the U.S. Embassy, and envoys of other permanent members of the U.N. Security Council to brief them on the explosion and seek their backing for a Lebanese complaint made last week against Israel's actions in the south.

The bomb exploded in the second floor of the husseiniyeh, a religious meeting place and lecture hall, as Saad and his aides were meeting with a group of men from the village. Reports from the scene said the side of the building was blown out.

Television film from Maarakeh showed hundreds of women thronging its streets, waving clenched fists and wailing, and young men atop the damaged building, beating their chests.

Israeli military officials said the explosion was caused by a car bomb, but Timur Goksel, spokesman for the U.N. force stationed in southern Lebanon, told Walsh this was impossible because the blast occurred on the second floor.

Goksel said U.N. units rushed evacuation helicopters and 12 medical teams to Maarakeh, and later sent heavy construction equipment to clear away the rubble from the explosion.

Villagers in Maarakeh said the bomb went off during a meeting on setting up distribution of relief supplies in the village following the Israeli raid, The Associated Press reported. Among the dead, authorities said, were Dr. Khalil Atwi, a Tyre physician, and Ahmed Rouhiyeh, head of the government-run social relief agency in Tyre.

Both Saad and Jeradi had recently made strong threats of attacks against the Israelis.

Saad told Reuter last week that Israeli practices were pushing the area toward radicalization. "If they attack this village or other villages again, we are going to attack Israel inside Israel," he warned. "We have the means and God's power to do it."

Before Saturday's search of Maarakeh by Israeli Army units, Jeradi was quoted in the Israeli press as saying that he would elude the Israeli forces and that "we promise them the Israelis four suicide raids a week. We have huge amounts of explosives and suicidal volunteers."

Israeli Defense Ministry spokesman Uri Lubrani, denying Israeli involvement in today's explosion, told Israel's state television: "I have no doubt we are talking about an internal power struggle within the Amal movement that is finding expression through means foreign to us."

Some Lebanese, however, harshly attacked Israel and, in some cases, the United States.

"It looks from these practices as though Israel has lost its nerve and panicked," Prime Minister Rashid Karami, speaking of general Israeli occupation actions, told reporters.

House Speaker Hussein Husseini, a Shiite from the Bekaa Valley, praised southern Lebanese fighting Israeli occupation, saying, "You have shown that you are the flame that can never be extinguished."

Sheik Mohammed Chamseddine, vice president of the Higher Shiite Council, called on Lebanese Christians to "join the resistance movement for the liberation of south Lebanon."

Hezbollah, the pro-Khomeini party that represents the Iranian-inspired militant fringes of Shiism, said, "One should not forget that America and Israel are behind what has happened to our people in Maarakeh."

Walsh reported from Jerusalem:

Touring the southern Israeli city of Eilat, Peres conceded that the get-tough policy against southern Lebanese "may have a detrimental effect in the long run." But, he added, "an army must defend itself, and the country must allow the army, both soldiers and commanders, to defend themselves. The Shiites know that we are on our way out of Lebanon."

Peres said he had been "quite shocked" to hear reports that Gemayel, visiting Sidon shortly after Israeli troops withdrew from that city last month, had urged Lebanese to "kill Israeli soldiers."

"This big hero," Peres said of Gemayel. "What incredible nerve. Therefore, the IDF [Israeli Defense Force] will on the one hand perform the mission assigned to it -- to withdraw from Lebanon in an orderly fashion, without fear or panic -- and on the other hand it will defend itself, as an army may and must do."

In Metulla, on the Lebanese border, Rabin told local officials that the new Israeli policy in southern Lebanon was meant "to make clear to the Shiites that they have a choice. We both live in peace . . . but if there isn't peace, then neither side will have peace."