Sudanese authorities indicated today that they were prepared to use violence to break up an antigovernment demonstration and strike called by a number of professional associations for Wednesday.

Meanwhile, several thousand citizens attended a downtown rally organized by the Sudanese Socialist Union, the country's sole legal party, in support of President Jaafar Nimeri's embattled government.

Some participants said they had been trucked or bused to the rally by government order and threatened with punishment if they failed to comply. Many apparently invoked the 100-degree heat as an excuse to wander away from the two-hour rally, which heard a message from Nimeri. He is now on a visit to the United States.

Orators and party slogans and placards criticized the Communists but concentrated their fire on the Moslem Brotherhood. That Islamic fundamentalist group was initially blamed for last week's antigovernment demonstrations and remains unpopular with many less orthodox Sudanese Moslems for having persuaded Nimeri to impose sharia, or koranic, law 18 months ago. The government recently has moved against the brotherhood and its leaders.

Young men armed with staves and hoe handles distributed by the Socialist Union were much in evidence and one orator warned that the party would use force to stop the professionals' demonstration.

The fate of that demonstration and the general strike appeared far from certain. A number of professional leaders were arrested yesterday. The engineers, who control the water supply, were reported to have decided not to join the other professional associations in the protest.

Striking doctors and lawyers are counting on bank staff, insurance agents and judges to take part in the demonstration.

But some observers suggested that the professional associations do not carry enough weight by themselves to achieve their goal of forcing Nimeri's resignation unless industrial workers from northern Khartoum join them.

In an effort to defuse resentment over rising prices and shortages, the government announced a major increase in salaries to coincide with the unfreezing of aid worth $180 million in Washington.

Opposition sources distributed copies of a three-page memorandum presented to Vice President Bush during his visit here last month calling for Washington to freeze all but humanitarian aid and condemn the Nimeri government "in the strongest, clearest terms" for human rights violations.

Signed by "concerned Sudanese academics and professionals," the memorandum criticized the "absence of any real or effective political participation . . . despotism and the denial of all democratic rights of any fundamental freedoms of speech, association, travel or assembly."

"There is an increasing recognition among Sudanese that the United States provides the only lifeline that is left for this decaying government," it added.

"When the Sudan is able to independently decide its destiny," it went on, "the criteria for its relationship with other states will ultimately depend on whether they supported a crumbling dictatorial government of soldiers of fortune or, instead, they furthered the cause of freedom, democracy or justice."

Meanwhile, economists studying the announcement of resumed U.S. aid said they expected major aid donors to meet soon in an effort to pledge $400 million to meet Sudan's deficit in the balance of payments this year.

They also expressed the hope that Sudanese authorities have been prevented from entering into onerous barter deals mortgaging the cotton crop to buy oil at well above world prices.

However, they noted that the announced pay increases appeared to be substantially larger than those the authorities had promised in negotiations with the International Monetary Fund.