Syrian opposition supporters have mounted noisy protests outside many embassies in recent months as the government of Bashar al-Assad has tried to put down unrest with what observers say has been a bloody crackdown.
Amnesty said embassy officials had filmed and threatened some of those involved in protests outside Syria, and that in some cases relatives in Syria had been deliberately targeted for harassment, detention, torture and outright disappearance.
"Expatriate Syrians have been trying, through peaceful protest, to highlight abuses that we consider amount to crimes against humanity - and that presents a threat to the Syrian regime," said Neil Sammonds, Amnesty International's Syria researcher.
"In response the regime appears to have waged a systematic - sometimes violent - campaign to intimidate Syrians overseas into silence. This is yet more evidence that the Syrian government will not tolerate legitimate dissent and is prepared to go to great lengths to muzzle those who challenge it publicly."
Syrian officials have generally denied reports of human rights abuses, with the Assad government saying it has no choice but to restore law and order and avert chaos.
The group said it had documented cases of more than 30 activists in eight countries -- Canada, Chile, France, Germany, Spain, Sweden, Britain and the United States -- who had faced some form of direct intimidation.
In many cases, those protesting outside Syrian embassies complained they had been initially filmed or photographed by officials and then received phone calls, e-mails and Facebook messages warning them to stop.
In some cases, those contacting them openly admitted they were embassy officials, demanding they stop any kind of political action and threatening a variety of consequences.
Naima Darwish, a Syrian protester living in Chile, says she was contacted directly by an embassy official who asked to meet her after she set up a Facebook group to organise a protest at the Santiago embassy.
"He told me that I should not to do such things," she told Amnesty. "He said I would lose the right to return to Syria if I continued."
One protester in Spain, Imad Mouhalhel, said his brother Aladdin had been detained in Syria for several days in July, had been shown photos and videos of protests outside the Madrid embassy and asked to identify Imad.
Aladdin was then briefly released before being detained again in August and apparently forced to phone Imad to tell him to stop his political actions. Aladdin had not been seen since, Amnesty said, expressing "grave fears" for his safety.
The rights group said Western governments had been far too slow to take action to rein in Damascus' diplomats.
"We look to host governments to act on credible allegations of abuses without waiting for formal complaints," said Amnesty's Sammonds.
"Many of the people we have spoken to are too scared of what could happen to them to make formal complaints with the police. We would expect that any official found responsible for such acts should be prosecuted, or - if diplomatic immunity prevents that - asked to leave the country."