DODOMA, Tanzania — Escorted by Western diplomats, Tanzania's main opposition candidate Tundu Lissu left the country for Belgium on Tuesday after rejecting a presidential election he asserted had "widespread irregularities" and was a "butchery of democracy."

"I am not fleeing the country," said Lissu, the survivor of an assassination attempt in 2017 who returned to Tanzania this year from Belgium to challenge populist President John Magufuli, who won a second term. "I am only going to explore different platforms to reclaim justice, democracy and dignity of Tanzanians."

Lissu was escorted at the airport by the U.S., German and Belgian ambassadors, and he posted video of his departure. Lissu's opposition CHADEMA party said he will be in Belgium for medical treatment.

The opposition leader had been housed at the German embassy since Nov. 2, when he was released by police after hours of interrogation. He and other opposition leaders had urged citizens into the streets for an "endless peaceful demonstration" over the Oct. 28 election, in which widespread fraud was alleged and many observers were barred. But police blocked the protest.

The United States and others noted credible allegations that called the vote's results — and the East African country's democratic ideals — into question. Allegations included the rejection of thousands of election observers, a massive slowdown in internet and text-messaging services and ballot box stuffing.

Magufuli later noted "a few challenges" around the election but called it generally peaceful. The government has denied any cases of intimidation.

Several opposition leaders were arrested or beaten or both around the vote. The chair of the CHADEMA opposition party, Freeman Mbowe, and two other CHADEMA leaders were charged with "terrorism-related offenses."

On Tuesday the United Nations human rights chief, Michelle Bachelet, said in a statement she was "disturbed by reports of continued intimidation and harassment against opposition leaders and members in Tanzania" after the vote.

The statement added: "Reports indicate that at least 150 opposition leaders and members have been arrested since 27 October in mainland Tanzania and in Zanzibar. While most have been subsequently released, at least 18 reportedly remain in custody."

She said those detained for exercising their human rights should be released.