More than 3,000 people have been arrested in Ethiopia, since disputed elections last month, human rights workers say.
Human rights workers say they have no access to the detention centres
The arrests have intensified since protests were violently suppressed last week, said an official with the Ethiopian Human Rights Council (EHRCO).
At least 36 people died when security forces fired at stone-throwing protesters, alleging fraud.
The European Union has called together political leaders to defuse tensions.
All parties signed an agreement on Friday to undertake a joint probe into complaints of voting irregularities.
But the opposition alleged the government could not be trusted - though it later withdrew the comments.
Observers say there is still considerable tension, as the government decides whether it can live with the outcome of an election that has apparently left it with considerably reduced authority.
The EHCRO official, who wished to remain anonymous, told BBC News that they had not been given access to detention centres.
He said that EHCRO researcher Chernet Taddesse had been arrested when he tried to investigate.
Meanwhile, shops and cafes have reopened in the capital, Addis Ababa, after being closed following last week's violence.
Blue Lada taxis have also returned to the streets after a five-day strike, the AFP news agency reports.
The government denies having placed under house arrest the main opposition leader and another senior official, saying that some opposition figures had been put under surveillance as a precautionary measure.
Coalition for Unity and Democracy leader Hailu Shawel and senior official Lidetu Ayalew were reportedly placed under house arrest on Saturday.
Journalists who went to Mr Hailu's home were beaten by police with fists and clubs and had their cameras confiscated, witnesses said.
"I am not allowed to leave my residence, my wife and maid are not allowed to leave and no-one is allowed to visit. I am virtually in prison," said Mr Hailu, according to Reuters news agency.
Information Minister Bereket Simon said the restrictions had been put in place because the men were a threat to state security.
"Immediately after the rejection of the agreement, the government observed new activities from CUD aimed at inciting more violence," he said.
Journalists were attacked at the opposition leader's home
The CUD denied reneging on the agreement.
"We have stated that we will abide by that agreement, but in order to implement it our members have to be released and our leadership allowed to move around, otherwise we cannot function," said Berhanu Nega, the coalition's vice-chairman.
Prime Minister Meles Zenawi defended Wednesday's security crackdown, saying that though he regretted the loss of life, "things were beginning to get out of control".
Friday's agreement - signed by both the ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) and the two main opposition coalitions - was aimed at ending the violence.
They also agreed to let the electoral board investigate more than 300 complaints in the 547 constituencies.
Three weeks after polling, final election results have not been declared.
According to provisional election results, the EPRDF and its allies have won 320 seats so far, giving it a majority in the 547-member parliament.