By Matthew Wells
BBC News, New York
Midtown Manhattan was brought to a brief standstill early on Thursday morning as police sealed off the blocks around the British consulate.
Michael Bloomberg said the motive was a mystery
There are few clues as to who did it, or why, but what authorities can say is that two homemade grenade-type devices - one apparently shaped like a pineapple, and the other like a lemon, according to one police officer - exploded shortly after 0330 (0730 GMT).
They had been placed inside a large cement flower tub, close to the doorway of the featureless and rather ugly office block which is home to the British Consulate on Third Avenue.
Nobody was injured, and the damage was slight. A large pane of glass was smashed by the blast, and a chunk of concrete was missing from the lip of the tub.
Anyone walking by at that moment would certainly have suffered injury.
Reporters were kept around 100m back from the building, which is also home to several stores on the ground floor, and more importantly perhaps, to several offices owned by foreign companies.
The British Consulate occupies the 9th and 10th floors - well away from the small blast area.
The early assumption is that whoever did this, knew the consulate was there, and was making some kind of protest or statement to coincide with voting today across Britain.
But New York's unflappable mayor, Michael Bloomberg, sought to curb speculation when he spoke to reporters at the scene.
"We at this point, have absolutely no knowledge of what the motive was... It is true the British Consulate is in that building, but I don't think anybody should jump to conclusions," he said.
He said there were no useful eyewitnesses, there were no suspects yet, and they had no leads beyond surveying CCTV footage of the area as fast as possible and doing more forensic tests.
"It's quiet in the city today. The British Consul-General is here - he of course is worried about the security. We have assured him that we will continue to provide plenty of protection," he added.
The city's police commissioner, Ray Kelly, was standing alongside the mayor.
He said the explosive devices were of amateur construction, and were not detonated using any timer.
The "novelty grenades" in question are easy to buy at any downtown novelty store. Some kind of black powder had been packed into them.
The two men, who are charged with keeping New York safe at all times in the wake of the 11 September 2001 attacks, said they were not taking the incident lightly.
The city has major counter-terrorism resources at its disposal, and they would be fully deployed.
The scene of the blast was cordoned off for hours
Walking away from the cordoned-off consulate towards downtown, it was clear that New Yorkers working in the neighbourhood were unperturbed by the temporary inconvenience.
"It's always worrying, but this is New York - stuff happens," said one man.
There had been plans to throw an election night party at the Consulate this evening attended by some of New York's best known British ex-pats.
The mayor said he hoped staff would join the rest of the neighbourhood's businesses in opening for business as soon as possible.
However small-scale this attack is, it is the first time that anything like this has occurred for a long while in Manhattan.
British officials - along with New York's counter-terrorism specialists - will be hoping to clear up exactly what happened and why, as soon as possible.