Iran has executed an average of almost two people a day in the first six months of this year, human rights groups have warned.

The sharp escalation in the use of capital punishment comes at a time when the Islamic regime is fighting to prevent pro-democracy movements similar to those that have been sweeping across the Middle East from taking hold in the country.

Human rights groups that have been carefully monitoring the rate of executions in Iran said the authorities had launched a fresh campaign of secret and mass hangings of prisoners in the provinces.

According to Amnesty International, Iran has acknowledged the execution of 190 people from the beginning of 2011 until the end of June but at least 130 others have also been reported to have been executed.

Iran Human Rights (IHR), an independent NGO based in Norway, told the Guardian it had recorded 390 executions since January, including two death sentences carried out on Thursday.

The International Campaign for Human Rights in Iran (ICHRI), a US-based non-government organisation, said its records showed 320 executions – a combination of those announced by the regime and those that have taken place in secret.

Hadi Ghaemi, executive director of the ICHRI, said: "The sharp rise of executions in Iran is a clear message that the state has no hesitation in using violence and applying it, no matter how arbitrarily, in holding on to power."

According to Ghaemi, Iranian officials are using execution as a means of intimidation to prevent popular discontent as the country heads towards the second anniversary of the unrest in the aftermath of its disputed 2009 presidential election, which gave president Mahmoud Ahmadinejad a second term in office.

Iran says executions are related to drug trafficking; it is a neighbour to Afghanistan, a leading producer and supplier of the world's drugs. But independent observers have questioned the veracity of the claims. At least two political activists have been identified as among those hanged in the first half of the year.

Iran has been proud of its fight against drug trafficking and has often been applauded by foreign governments for its achievements, but activists say hanging criminals is a cover-up for a wider purpose.

"Recently, Iran's state television showed a group of armed forces raiding the house of some people who they described as criminals but, by depicting such violence on national TV, Iran is showing its ability to exercise violence in any circumstances," said Ghaemi.

IHR said it had received credible reports from Karaj, a city west of Tehran, that 25 people were hanged in a secret mass execution on Sunday in Ghezel Hesar prison on charges related to drug trafficking.

Last week, the ICHRI reported that 26 inmates were executed in Vakilabad prison in the eastern city of Mashhad on 15 June.

An earlier report by Amnesty International in April warned against Iran's use of public executions, in which authorities publicly hang convicts from a large crane. The report said that this year 13 people had been hanged in public – including two juveniles – compared with 14 in the whole of 2010.

In June, the UN human rights council appointed former Maldivian foreign affairs minister Ahmed Shaheed as a UN special rapporteur investigating Iran, but several Iranian officials have signalled he will not be allowed to visit the country.

Mahmood Amiry-Moghaddam, a spokesman for the IHR, said: "The biggest challenge for Shaheed is to make Iran co-operate and to get access to Iran, especially in order to highlight these executions.

"We think that Iran will put pressure on the families of those who are executed not to co-operate with the UN but our request from Shaheed is to investigate these secret and mass executions."

Last year, 252 people were executed according to Iranian officials, but human rights groups say 300 more executions went unacknowledged by the state.