Octavio Ortega says he was attacked and illegally detained by officers after protest against Chinese-funded project

A leading opponent of Nicaragua’s transoceanic canal says he was brutally beaten with batons, rifle-butted in the face and illegally detained by police after he organised a protest against the inauguration of the Chinese-funded project.

Photographs of his injuries, including a black eye and badly bruised back, have been published on social media.

In his first interview with the foreign media since he was released on 30 December, Octavio Ortega, president of the Fundemur NGO, told the Guardian he was physically abused before he was put in jail.

“My cellmates had to lift me up and help me so I could relieve myself,” he recalled. “The first night I couldn’t sleep because I couldn’t lie down. I spent the first four days in a bad way.”

He said police demanded to know whether he was working for the CIA and that he only received the right medication for his hypertension and diabetes on the fifth day of his seven-day detention.

The protest, detention and beatings highlight the tensions surrounding the canal – the biggest infrastructure project in the country’s history – which aims to rival Panama as a trade route across the central American isthmus.

Ortega has been an outspoken opponent since the plan was approved by the parliament in 2012. He says the project was not adequately debated and will destroy the environment, particularly Lake Nicaragua, the largest body of freshwater in Central America.

He has rallied thousands of landowners who are worried that their property will be expropriated by the government for the 174-mile channel and its subsidiary projects, including an airport, tourist resort and oil pipeline, on behalf of the company that will build and operate the project, Hong Kong-based HKND.

In the most recent of half a dozen demonstrations, he joined a roadblock across the Pan-American Highway on 23 December to protest against the groundbreaking ceremony for the $40bn project the previous day. Ortega insisted the demonstration was peaceful, blocking the traffic every half hour to explain to drivers that “we don’t want to sell our properties, we have never put our properties up for sale, and we don’t want Lake Nicaragua to be destroyed”.

But police cleared the roadblock – and another protest at El Tule – by force. At least 28 protesters were detained, including eight women and five minors. Ortega said six riot police attacked him and then he was taken to Managua’s judicial auxiliary directorate.

Other detainees confirmed a similar pattern. Danilo Lorío, a community leader from El Tule, told the Nicaraguan newspaper La Prensa that “they were beating us all the way on the bus”, although not in the prison cells.

According to Nicaraguan law prisoners can only be held for 48 hours before being released or taken before judicial authorities, but Ortega was kept almost seven days in what he describes as a “four-by-four cell, with two concrete bunks, a hole to relieve yourself and water for just one hour in the morning and another hour at night”. He believes he was held longer so that his injuries would have time to heal and not look so bad.

Police chief Aminta Granera has accused Ortega of threatening to blow up a petrol tanker during the protest and only failing because “fortunately his lighter did not work.”

Ortega has not been charged and vehemently denies targeting the tanker, which was parked near the site of the protest. “How can you set fire to a tanker using a lighter? And anyway I don’t have one because I don’t smoke,” he said.

With the project set to accelerate this year, the government and HKND now face the challenge of mollifying landowners and other critics.

Wang Jing, the president of HKND, has promised compensation will be paid according to market principles in a fair and transparent way. “Nobody is going to touch an inch of anyone’s land unless there is an agreement and the owner is satisfied with the payment,” he said last week.

Ortega, however, is unimpressed and undaunted. “We’re going to carry on with our peaceful, civic struggle for our rights … we want to continue channelling the response in a peaceful way so that this government understands there are laws to comply with and international treaties to respect,” he said.