CAIRO (AP) — Sudanese professional and opposition groups held protests Thursday in at least a dozen major cities, including the capital Khartoum, where hundreds braved tear gas and sometimes live fire as they marched toward the presidential palace to deliver a written request that President Omar al-Bashir step down, according to activists.

The protests are part of a wave of discontent over a failing economy that has transformed into demands for the resignation of the autocratic al-Bashir, an Islamist who has run the country for nearly 30 years but brought little improvement to his people.

"Day after day, the number of demonstrators is increasing. Today we will gather in the thousands and tomorrow we will reach the millions," said Reem, a 35-year-old receptionist demonstrating in Khartoum. "I will not stop until we achieve the change that gives me a decent life, a job and the salary I deserve."

Mohammed Yousef, a spokesman for the Sudan Association of Professionals, said protesters were prepared to continue to press their grievances while remaining "patient and wise."

"The people of Sudan are known for being particularly determined, stubborn, and for playing the long game. They are not hot-headed, nor do they despair easily," he said.

The government crackdown has been harsh over the past month, with rights advocates reporting the use of excessive force by police and Amnesty International accusing security forces of firing tear gas and live ammunition in and around hospitals. At least 40 people have been killed in the clashes, according to rights groups, but the government has acknowledged only 24 deaths.

The U.N. High Commissioner for Human Rights Michelle Bachelet called such reports "credible" and "deeply worrying," urging the government to ensure citizens' rights to freedom of expression and peaceful assembly, regardless of their political affiliations, according to Sudan's international treaty commitments.

"A repressive response can only worsen grievances," she said.

Bachelet said the U.N. would be ready if asked to deploy a team to Sudan to advise authorities "and help ensure they act in accordance with human rights obligations."

Hundreds of demonstrators aged in their 20s and 30s gathered in the back streets around the presidential palace, calling for a peaceful uprising and the downfall of al-Bashir, while men in civilian clothing carrying assault rifles — some wearing face masks — blocked the main approaches, activists said.

Videos posted online — the only means to communicate freely in Sudan — showed several marches in the hundreds across the country, with activists claiming nearly 2,000 demonstrators were in central Khartoum and two dozen of its neighborhoods held their own protests. They also circulated photos showing several demonstrators purportedly injured by live fire.

Security forces arrested several journalists near the palace who were reporting on the march, they added, while police fired tear gas to disperse crowds elsewhere in the city's central Arab Market area. Demonstrations were also reported in Sudan's central Gezira region as well as the southeastern city of Gadarif and the Western region of Darfur.

Activists spoke on condition of anonymity as they weren't authorized to brief reporters.

The Thursday rallies took place in the cities of Khartoum, Madani, Sinnar, Rafaa, Atbara, Abu Jubayhah, Gadarif, Um Rawaba, Dowim, Al-Obeid, Port Sudan and Geneina — after calls were made for demonstrations there, as well as several others.

Separately, a teachers' union announced a weeklong strike in all schools beginning Sunday.

Sudan's economy has stagnated for most of al-Bashir's rule, but its recent lows have been dramatic, with surging prices and a plummeting currency that prompted the protests. He has also failed to unite or keep the peace in the religiously and ethnically diverse nation, losing three quarters of Sudan's oil wealth when the mainly animist and Christian south seceded in 2011 following a referendum.

Bashir, who is also wanted by the International Criminal Court for genocide in Darfur, has said those seeking to oust him can only do so through elections.