Opposition demonstrators today defied stringent martial law provisions for the third straight day here amid indications that their 11-month struggle to bring down Shah Mohammad Reza Pahlavi may be entering a crucial phase.

Pouring rain appeared to have depleted the ranks of opposition street demonstrators braving army patrols tonight after the 9 p.m. curfew. But sporadic shots were heard in several neighborhoods and thousands shouted "Death to the shah" and "Allah is great" from the safety of their roofs.

Much of Tehran's normally bustling downtown commercial district was closed, with the exception of food stores. The Light traffic reflected that, although today was quieter than Friday and Saturday's violent confrontations, the worst trouble is yet to come now that the Shiite Moslem month of mourning, Moharram, has begun.

Religious fervor traditionally runs high during Moharram, which is expected to sharpen religion-based opposition to the shah's liberalizing reforms. In addition, the shah's military government has banned customary Moharram processions, a ban some Shiite leaders have vowed to defy.

Until the newest outbreak of street violence and tension began Friday -- the beginning of Moharram -- most Iranian and foreign observers had expected trouble only next weekend.

Next Sunday marks the anniversary of the martyrodom of Imam Hossein, the prophet Mohammed's grandson and founder of the Shiite sect of Islam followed by more than 90 percent of Iran's 35 million inhabitants.

Ayatollah Ruhollah Khomeini, the Parisbased religious leader symbolizing Iranian opposition to the shah, called once again last week for increased insurrection, underlining the religious motives of many rebels.

In leaflets distributed here, he said "torrents of blood" will flow on Ashura, the 10th day of Moharram mourning.

"You will be remembered for your sacrifices," he added in a gesture bestowing religious blessing on victims of the violence.

It was on Ashura in 1963 that bloody rioting broke out that led to the exile of Khomeini in Iraq, where he stayed until moving to Paris this fall.

Khomeini clearly seemed to be calling for continued defiance of the month-old military government, saying: "Blood will triumph over the sword."

An indication that his call for sacrifice is being heeded was provided by the white shroud worn by many demonstrators.

That is the traditional Shiite garb of mourning and according to local tradition, it indicates its wearer is prepared to die for his faith.

For the first time since the crisis began last January, the government-controlled radio and television have charged that demonstrators have begun using weapons ranging from hand guns to Molotov cocktails in what observers fear could be the start of open insurrection.

Gen. Gholam Reza Azahri, the prime minister, said the army had seized some weapons. Diplomats for months have reported arms smuggling into the poor neighborhoods of southern Tehran and do-it-yourself pamphlets have been found explaining how to make Molotov cocktail firebombs.

Meanwhile, the government and opposition differed over the casualty toll from the first two days of the renewed confrontation. Although the government maintains only 12 persons were killed on Friday and Saturday in the capital, the opposition has said the number of victims ran into the thousands.

At the main Tehran cemetery, informed sources said 305 persons were buried Saturday, sharply above normal.

The government also was running into continued economic problems, even in sectors that had gone back to work recently after long strikes.

Oil production, for example, which returned to 6 million barrels a day after a prolonged strike last month, declined by about 500,000 barrels a day as workers of the Iran-Pan American offshore firm started a walkout.

The government has again closed schools, Iranian and foreign, and many members of the foreign community are sending their families abroad to avoid interruptions in their children's education.

As has become standard operating procedure in this slow-motion revolution, the American and other embassies once again have warned their citizens to stay off Tehran streets.

Indicative of growing nervousness among the Iranian middle class has been the quadrupling of visa requests -- to 700 to 800 a day this week -- at the U.S. Embassy. Contributing to the sense of unease has been the month-old closing of newspapers. Journalists have refused to work unless the military government agrees to accept a free press.

The government-controlled radio and television are distrusted by many Iranians and authorities have resumed jamming the British Broadcasting Corp.'s Persian language broadcasts, which long have been a source of uncensored news here.

Meanwhile, opposition sources reported a continuing pattern of demonstrations and violence in the provinces. There was no official confirmation.

The opposition reported that heavy shooting had taken place in the Caspian Sea town of Amol and in the major city of Isfahan, where as many as six persons were said to have been killed.

[The Atlanta Journal-Constitution said Sunday that U.S. intelligence officials believe the Soviet Union has been financing conservative Moslem opponents of the shah through the French and Italian Communist parties, United Press International reported.]

[Sources said intelligence officials have reported to President Carter that they believe the shah's government has no better than an even chance of surviving, the newspaper reported.]