The 49 Americans being held hostage by radical Islamic students at the U.S. Embassy here ended their third week in captivity today with no sign of a relaxation of tensions that could lead to their release.

Ayatollah Ruhollah Khomeini, in a new attack on the United States today, accused President Carter of using "medieval logic and jungle law" in his demands for release of the hostages. He also accused the United States "and its corrupt colony Israel" of attempting to "take over" the Great Mosque in Mecca.

In a message broadcast to 16 Arab liberation movements with headquarters in Algiers, the 79-year-old religious leader said U.S. threats of military intervention were the hostages harmed, amounted to "the law of the jungle" which ignored "all humanitarian and international values."

Khomeini said one of President Cater's "grave mistakes is not to understand the depth of the Islamic movement." He called on "Moslems of the world" to "stand up (and) invoke the might of God and defend Islamic and national aspirations."

Despite such tough talk, Khomeini reportedly approved a visit to the hostages by Rep. George V. Hansen (R-Idaho), and Sean MacBride, former Irish foreign minister and winner of both the Nobel and Lenin peace prizes.

The students apparently refused to act on the advice from Khomeini, however, and Hansen and MacBride were unable to enter the embassy compound.

Observers now fear that few developments can be expected before the referendum on the new constitution is completed a week from Monday. They reason that Khomeini wants to capitalize on the national unity created by the current anti-U.S. campaign to secure a massive turnout and "yes" vote for the controversial document, which would strengthen religious control of the country.

Since the hostages were seized Nov. 4, dissidents of all political beleifs have fallen into line behind Khomeini.

In this context, diplomats representing the nine European Common Market countries once again saw acting Foregin Minister Abol Hassan Bani-Sadr today to press for better conditions of detention and regular access to the hostages by a neutral body.

Meanwhile, signs of timid discontent with the new revolutionary trend have begun to surface.

An editorial in the newspaper Kayhan noted that rumors were suggesting that the anti-U.S. movement "was engineered by the ruling Revolutionary Council with premeditation to cover up for its executive flaws."

Accusing the council of having failed to keep promises to purge the administration made when prime minister Mehdi Bazargan's government was forced to resign nearly three weeks ago, the editorial accused its members of electioneering and demagoguery.

"Pessimists can propagate the view that by fanning the flames of the embassy hostage-taking, the Revolutionary Council is coverning up its own lack of efficiency," the editorial added.

Bazargan himself was quoted today in another newspaper, Bamdad, criticizing the clergy's now dominant role in politics in the clearest terms.

"The clergy," he said, "is opposed to management and wants to administer the country with its own system."

The French-trained engineer said, "The same ones who oppose Western civilization have their talks broadcast by the radio and television . . .

"When they want to go to Qom, [the holy city 80 miles to the south where Khomeini lives] "they go in helicopters," and when they fight, he said, they use modern weapons.

"Whether we like it or not, we are in the 20th century," he added, "we cannot forget the connections and build a Great Wall of China around us."

"I said 'just let the shah leave the States and I think the embassy question will end in a peaceful way,'" PLO spokesman Mahmoud Labadi quoted himself Friday as having told a reporter.