A number of opposition leaders have been arrested just two days before parliamentary elections in the oil-rich former Soviet republic of Azerbaijan.
Pro-government marches have been allowed in a central square
Most were soon freed, but the campaign manager for the Democratic Party was still in custody, party officials said.
President Ilham Aliyev has promised free and fair elections, but the country has a history of vote fraud.
Sunday's vote will be the first since Mr Aliyev replaced his father, a Soviet-era strongman, in 2003.
Police raided Democratic Party headquarters on Thursday and detained campaign manager Faramaz Javadov, party officials said on Saturday.
An interior ministry spokesman confirmed the raid had taken place.
"We had a lead on some weapons that may have been stored there," ministry spokesman Sadiq Gezalov told the AFP news agency.
Complaints of intimidation
Opposition activists and international observers have complained the government has been intimidating, arresting and harassing opponents.
Three leaders of an opposition youth movement were jailed two months ago, accused of planning an armed revolt.
At least 12 government officials, including two cabinet ministers, have been sacked and arrested on charges of planning a coup.
The opposition hopes for a repeat of Ukraine's Orange Revolution
The head of the Democratic Party, former parliamentary speaker Rasul Guliyev, was detained in Ukraine last month as he attempted to return from self-imposed exile in the US.
Mr Guliyev fled the country a decade ago, accused of corruption charges which he says were politically motivated.
The opposition has been denied permission to rally in a central square in the capital Baku, while the governing New Azerbaijan Party held a demonstration there on Friday.
The US, which played a key role in construction of a new oil pipeline from Azerbaijan to the West, has called for free and fair elections.
"Free expression of the will of the people through such elections is vital to Azerbaijan's future and to a strengthened US-Azerbaijani relationship," US state department spokesman Sean McCormack said.
President Aliyev has introduced a number of voting reforms, including inking the fingers of voters to prevent them casting multiple ballots.
His father Heydar Aliyev ran the country for 10 years until his death in December 2003.
About 2,000 candidates - including Ilham Aliyev's wife - are standing for 125 seats in parliament.
Azerbaijan sits at a strategically vital point on the Caspian Sea between Iran, Russia and Turkey.
US Defence Secretary Donald Rumsfeld has visited a number of times in the past few years, prompting speculation the US is considering stationing forces in Azerbaijan.