Police in New York have arrested a US citizen and a Pakistani national on suspicion of plotting to blow up a subway station.
Kelly said the two arrested men did not have explosives
The New York police chief said the two men had surveyed one of the alleged targets close to the venue of next week's Republican Party convention.
Police chief Raymond Kelly said they had a clear intention to cause damage and kill people, but had no explosives.
New York police earlier held 264 people rallying against President Bush.
They were among 5,000 cyclists who stopped traffic around Madison Square Garden, where the Republican convention will be held.
The BBC's Jeremy Cooke in New York says the scale of the unrest reinforces the uncompromising tone taken by New York's police chief Ray Kelly, who has publicly warned that demonstrators breaking the law will be taken into custody.
'Hatred for America'
Mr Kelly said the two man - identified as US citizen James Alshafay, 19, and 21-year-old Pakistani Matin Siraj - had surveyed a number of sites including Herald Square subway station, close to Sunday's Republican convention,
Thousands of cyclists took to the New York streets
He said they were not thought to be part of any terror group and had no explosives in their possession, and there was no evidence to suggest the pair had focused specifically on the convention itself.
But the commissioner said it was "clear they had the intention to cause damage and kill people... They did not immediately have the means to do it."
"Their motive was generally hatred for America."
Mr Kelly added that the pair had been under police surveillance for a year and had been recorded discussing plans for an attack.
Earlier on Saturday, city police arrested 264 cyclists who took part in a protest in Manhattan.
Moving at snail's pace through the streets, they shouted anti-Republican and anti-Bush slogans.
A police spokesman said the protesters had caused massive disruption and endangered motorists.
In turn, the anti-war group United For Peace accused the police of needlessly escalating tension.
However, many protesters say they are determined to gather in Central Park on Sunday, despite bans imposed by the New York city authorities and the courts.
Our correspondent says as many as 250,000 people are expected to take part in a march which will file past the convention hall.