Rights group Amnesty International has urged China to disclose the number of prisoners it executes.
In its annual report on the use of the death penalty, Amnesty said some 714 people were known to have been executed in 18 countries in 2009.
But the group said the true global figure could be much higher, as thousands of executions were thought to have been carried out in China alone.
At least 388 people were executed in Iran, 120 in Iraq and 52 in the US.
Amnesty praised Burundi and Togo for abolishing the death penalty in 2009 and said that for the first time in modern history, no-one had been executed in Europe or the former Soviet Union over the year.
Beijing says it executes fewer people now than it has in the past, but has always maintained that details of its executions are a state secret.
However, Amnesty said that "evidence from previous years and a number of current sources indicates that the figure remains in the thousands".
It said the death penalty could be applied to 68 offences in the country, including non-violent crimes, with executions carried out by lethal injection or firing squad.
Many people were sentenced based on confessions extracted under torture and having had limited access to legal counsel, it said.
"The Chinese authorities claim that fewer executions are taking place," said Amnesty's Interim Secretary General Claudio Cordone.
"If this is true, why won't they tell the world how many people the state put to death?"
Since 2007, all death sentences passed in China have been subject to a mandatory review by a higher court, a process China says has reduced the number of killings carried out.
"However, as long as statistics on the use of the death penalty in China remain a state secret, it will be impossible to verify this claim and to analyse actual trends," said Amnesty.
Of particular concern to Amnesty were cases of those executed after political unrest in Tibet and Xinjiang, people sentenced to death for financial fraud and a British man, Akmal Shaikh, executed for drug smuggling despite his lawyer's claims he was mentally ill.
"The time is long overdue for China to fall into line with international law and standards on the death penalty and be open and transparent regarding its use of capital punishment," it said.
Amnesty said that by the end of 2009, there were 17,118 people on death row around the world, with 2,001 people sentenced that year.
But while 58 countries still had a death penalty in 2009, only 18 countries were known to have carried out executions.
It also said "commutations and pardons of death sentences appear to be more frequent" in countries which still pass death sentences, including more than 4,000 in Kenya in a mass commutation in August.
The group noted a sharp rise in executions in Iran in the eight weeks after political unrest following President Mahmoud Ahmadinejad's election victory in June 2009.
Iran was also criticised, along with Saudi Arabia, for carrying out executions of people convicted of crimes they committed while under the age of 18.
Saudi Arabia was reported to have carried out executions "at an alarming rate", with at least 69 people publicly beheaded in 2009.
The report also highlighted an increasing abolitionist trend around the world in recent years.
Both Burundi and Togo outlawed the death penalty in 2009, becoming the 94th and 95th countries to do so.
"The world is in reach of 100 countries declaring their refusal to put people to death," said Amnesty.
The group repeated its assertion that the death penalty was cruel, an "affront to human dignity" and often used disproportionately against the poor and marginalised.
It said the secrecy surrounding state executions in many countries was "indefensible".
"If capital punishment is a legitimate act of government as these nations claim, there is no reason for its use to be hidden from the public and international scrutiny," it said.