DAR ES SALAAM, Tanzania — As one of the leaders of an acrimonious doctors’ strike in Tanzania, Dr. Stephen Ulimboka was not entirely surprised when a group of armed men appeared, unannounced, at a meeting and arrested him. But when he saw that the car they were forcing him into had no license plates, fear truly hit him. 

American and Tanzanian flags lined a street in Dar es Salaam, the capital, on Sunday, ahead of a visit by President Obama
A black hood was thrown over his head. “You’re going to pay for what you’ve been doing,” Dr. Ulimboka recalled one of the men saying. “You can start praying to your God because there is no turning back.” 
They beat him for hours on that June night last year, first with their fists, then with metal rods. They pulled the toenails from both of his big toes. As he lay on the ground, he heard them discussing the best way to kill him: running him over with the car or giving him a lethal injection. He was unsure if he would live till daybreak. 

Tanzania has a reputation abroad as an island of stability in the often-chaotic region of East Africa. The country has been rewarded with praise and money from international donors, including the United States, which last year gave the country more than $480 million. 

President Obama arrives here on Monday to a country where human rights groups and the largest opposition party say episodes of intimidation and suppression of political opponents are growing. “The international community believes there is peace in Tanzania,” said Willibrod Slaa, the secretary general of the opposition party, Chadema. “There is fear, not peace.” 

Journalists have been attacked and in at least one instance killed while working. Last July, the government banned an independent weekly newspaper, Mwanahalisi, which had been reporting aggressively on Dr. Ulimboka’s kidnapping, linking the crime to the government. President Jakaya Kikwete denied any connection. 

“I don’t feel secure,” said Saed Kubenea, managing editor at Hali Halisi Publishers Ltd., which owns Mwanahalisi. “But I will fight.” 

The Committee to Protect Journalists, a nonprofit organization based in New York, urged Mr. Obama last week to raise the issue of freedom of the press when he meets with Mr. Kikwete on Monday. 

The political violence reached a new, unexpected level last month, when a hand grenade was thrown at a rally organized by Chadema in the northern city of Arusha, killing four people. No suspect has been identified, and the investigation is continuing.
At the party offices here in Dar es Salaam the other day, a fleet of motorcycles used for reaching isolated constituencies in villages accessible only by dirt road sat parked out back. Party officials placed a silver laptop on a table and showed a video from the rally in Arusha. 

In the footage, party leaders gave speeches from atop a truck with built-in speakers. Afterward, they descended into the crowd and began collecting donations. A blast sent people scattering. A handful of wounded and dead were frantically gathered and carried to the bed of a pickup truck that took them to receive medical treatment, leaving behind a blacktop slick with blood. 

“It is intimidation,” Mr. Slaa said. “The people will be afraid to go to the polling stations, and the active ones will have been eliminated.” 

Chadema officials have publicly claimed that the man responsible was either working with, or protected by, the police. They say the party will produce videotape proving their charge, but only after an independent commission has been named to investigate. 

Paul A. M. Chagonja, commissioner of police for operations, called the allegations “frivolous” and “unfounded,” and said the party was obligated to furnish law enforcement with any evidence in its possession. 

“The core function of the police is to protect the people,” Mr. Chagonja said. “We are not allied with any political party.” 

Tanzania, home to Mount Kilimanjaro, is a popular tourist destination for safaris in the Serengeti. The nation has been lauded for its ethnic cohesion, rising above the kind of tribal violence that rocked Kenya after that country’s elections in 2007. Although a church bombing in May, also in Arusha, raised concerns that religious tensions could rise, Tanzania is relatively free of sectarian strife. That is one reason Mr. Obama scheduled a visit here. 

 Yet the Tanzanian government has essentially remained in the hands of the same party since gaining independence half a century ago. Tanzania held its first multiparty elections in 1995, but the ruling party, Chama cha Mapinduzi, or Party of the Revolution in Swahili, has won the national elections each time since.

 Analysts say the very real prospect that voters will choose another party in the next election, in 2015, has rattled some members of the government, particularly those who are afraid that a new party in power could mean aggressive investigations and prosecutions. 

“I think there is a rear-guard element in ruling circles who have never accepted this,” said Jenerali Ulimwengu, a prominent Tanzanian journalist. “They haven’t been reined in by the political bosses because they are shaky and unsure.” The result, Mr. Ulimwengu said, “can be quite deadly, as we’ve seen over the past couple of years.” 

Abdulrahman Kinana, secretary general of the ruling party, known as C.C.M., said it was prepared to accept a defeat at the ballot box. “We were always ready to transfer power if the people decide,” he said, adding that C.C.M. won the country’s “free and fair elections” by reaching out more effectively to voters. He pointed to the dozen or so daily newspapers available here as evidence of a vibrant local news media. 

But the government “needs to tell us what happened to those people who were either killed or attacked,” Mr. Kinana said. “Most of these crimes have not gotten an explanation.” 

The men who kidnapped and tortured Dr. Ulimboka took him to a forest, where he was dumped into a hole about three feet deep, his arms and legs bound. He laid as still as possible, hoping the men would believe he was already dead. He waited for about half an hour after they left before struggling to free his legs. 

He walked toward the sound of a road, his hands still bound behind his back, the rope biting deeply into his wrists. There, he found help and was taken to a police station and later to a hospital. His kidneys were failing, and he had to be flown to South Africa for treatment. 

A year later, most of his injuries have healed, though he said that when he combed his hair, he felt the numb spots where his nerves had been damaged in the savage beatings. He does not fear for himself at a time when people are killed at public gatherings. 

“People,” Dr. Ulimboka said, “can just kill you anywhere.”