By Tiksa Negari and Giulia Paravicini

ADDIS ABABA (Reuters) - Police fired gunshots and teargas as thousands protested in Ethiopia on Wednesday over the treatment of a prominent activist, residents said, in a sign that the country's Nobel Prize-winning prime minister might be losing support among his powerbase.

More than a thousand supporters gathered in Addis Ababa outside the house of Jawar Mohammed, a media entrepreneur who organized protests that brought Prime Minister Abiy Ahmed to power last year, after police surrounded the building.

Protests quickly spread to the other parts of the capital and to the cities of Adama, Ambo, Harar and Jimma, residents said. Several people were reported to have been killed in Addis Ababa and Harar.

On Tuesday, Abiy had warned against media owners "fomenting unrest". That night, security forces surrounded Jawar's house and the government attempted to withdraw his security detail, Jawar told Reuters.

The next morning, a Reuters witness saw at least 400 young men from the Oromo ethnic group chanting support for Jawar and against Abiy, the winner of this year's Nobel peace prize. Around two dozen police officers stood nearby.

Abiy has won international praise for his sweeping political reforms but greater freedoms have lifted the lid on long-repressed tensions between Ethiopia's many ethnic groups.

Abiy must walk a delicate line between increasing political freedoms and reigning in strongmen building ethnic powerbases by demanding more access to land, power and resources for their groups.

Jawar, an Ethiopian-born U.S. citizen, is an activist from the Oromo ethnic group, the country's largest. Abiy is also an Oromo.

Jawar's wide reach - his Facebook page has 1.75 million followers - means he can quickly mobilize demonstrators.

Some Ethiopians have criticized him for using ethnically tinged language, but many young Oromo men consider him a hero who brought the political change that resulted in Abiy's appointment last year.

SHOWDOWN

At least 20 young men caught up in demonstrations on the outskirts of the capital were wounded and one was killed, a local businessman told Reuters by phone from Alert Hospital, where he said he had gone to help a wounded friend.

After the showdown in the capital, demonstrations spread to other cities, residents told Reuters.

In Adama, 90 km (50 miles) southeast of the capital, two residents said they heard gunshots amid protests in support of Jawar there on Wednesday afternoon. It was not immediately clear who fired the shots.

In Ambo, 100 km (60 miles) from the capital, police fired teargas and bullets at thousands of protesters, and at least four people were shot, two residents who spoke on condition of anonymity told Reuters.

There were demonstrations in the city of Jimma, 350 km (220 miles) from Addis Ababa, residents said.

Violence also broke out in parts of the capital. Reuters spoke to a 26-year-old protester wearing a T-shirt with Jawar's face on it as he lay on the floor of the Black Lion Hospital in Addis Ababa waiting to be treated.

"Police hit me repeatedly with iron bars and broke my left arm and my left leg," Gemechu Qitata said. He had been protesting in Jemo, a southwestern district of the capital.

Protests also erupted in Harar, 500 km (300 miles) east of Addis Ababa, where a police officer told Reuters that a peaceful protest became chaotic when demonstrators began burning tires and blocking roads. The officer, speaking on condition of anonymity, said some protesters were killed but he did not know how many or who had killed them.

Shimelis Abdisa, vice president of the Oromiya region, said what had happened on Wednesday was a "major mistake" and called for an investigation.

MEDIA NETWORK

Jawar, founder of the independent Oromia Media Network, returned to Ethiopia from the United States last year after Abiy come to power and the two have been photographed repeatedly together since.

On Tuesday Abiy issued a warning in a speech to parliament: "Those media owners who don't have Ethiopian passports are playing both ways," he said. "When there is peace you are playing here, and when we are in trouble you not here.

"We tried to be patient. But if this is going to undermine the peace and existence of Ethiopia ... we will take measures. You can't play both ways."

A spokeswoman for Abiy's office did not respond to requests for comment. Abiy met Russian President Vladimir Putin on Wednesday at a Russia-Africa summit in the Russian city of Sochi, according to the Twitter feed of Abiy's office.

Abiy came to power in April 2018. His reforms have opened up what was once one of Africa's most repressive nations, but also stoked violence along ethnic lines. Dozens, including the army chief, were killed during a foiled coup by a rogue state militia in the Amhara region in June.

Ethiopia is due to hold elections next year.

(Reporting by Tiksa Nigeri and Giulia Paravicini; Additional reporting by Kumerra Gemechu; Writing by George Obulutsa and Maggie Fick; Editing by Alex Richardson and Giles Elgood)