US president George Bush hailed 'the daring of our rescue workers' and the efforts of local authorities and emergency teams in New York and Washington.
Washington mayor Anthony Williams thanked district staff for 'staying calm, keeping our streets clear and co-operating to help us through this difficult time'.
In London, mayor Ken Livingstone pointed out that public sector workers had performed heroically and called on the capital to remain 'as vigilant as ever'. He met with emergency services to discuss what help could be offered to New York and the security implications for London.
Tower Hamlets LBC, which contains Docklands and part of the financial district, asked residents to be 'aware of security'.
There was concern speculation about who was behind the attacks could fuel racist tensions, following reports from the US of threats against Arabs and Muslims.
London boroughs, the Metropolitan Police, and community groups were working together to 'ease any potential community tensions'.
Bradford MDC member Inkthab Alan (Con) said he was stunned by the attack and hoped it would not set back community relations in the city.
He warned: 'It is too early to speculate. There are no suspects as yet. But if this is going to end in a religious war, we have to hope it is not brought to England.'
Local Government Association chair Sir Jeremy Beecham (Lab) said: 'Councils have a role to play in ensuring the reaction to the atrocities is not manipulated by extremists to promote negative stereotypes.'
He called on councils to issue joint statements 'expressing the horror of their constituents'.