Mr Blair flew in from Washington after talks with President Bush
Mr Blair arrived in the Belgian capital on Friday morning on the final leg of a hectic diplomatic mission which has taken him from Europe to the US and back in less than 48 hours.
On Thursday he held talks with President George W Bush in Washington and visited New York where he joined a remembrance service for the Britons who died in the World Trade Center disaster on 11 September.
Mr Blair will be a central figure at the summit, relaying the issues covered in his talks with Mr Bush to other EU leaders.
Along with the French President Jacques Chirac and German Chancellor Gerhard Schroeder, Mr Blair is expected to endorse the US stance in the aftermath of the suicide attacks.
"Essentially I hope to achieve two things: first of all a very strong and clear declaration of our support and solidarity for the United States and secondly, a series of substantial measures that we will take - both European and nationally," the prime minister said.
Mr Bush has warned Afghanistan to hand over Osama Bin Laden and his followers - believed by the US to be behind last week's attacks - or "share in their fate".
Iran visit
Earlier, it was announced that UK Foreign Secretary Jack Straw will visit Iran next week for discussions on the aftermath of the suicide attacks.
The country's government is said to be concerned about the possibility of military strikes against neighbouring Afghanistan where Bin Laden is thought to be hiding.
Mr Blair left London on Wednesday afternoon and visited Berlin and Paris before flying to the US.
At Friday's summit, the EU heads of state will discuss the counter-terrorism campaign and "take stock of the international situation", an official said.
They are also expected to endorse new anti-terrorist measures agreed by EU justice and interior ministers in Brussels on Thursday.
The 37 proposed measures include an EU-wide search and arrest warrant.
Mr Chirac and Mr Schroeder have told Mr Blair their countries would be ready to contribute forces if needed for a strike against Afghanistan.
Practical support
Mr Blair, who will be joined by Mr Straw, is planning to hold more meetings with individual EU government leaders after arriving in Brussels several hours before the summit begins.
The summit could expose how far individual EU governments are prepared to go in offering practical support to the United States on the ground or in the air for the war against "global terrorism".
Mr Chirac has already suggested that France "will not stand aside in a fight against a scourge that defies all democracies".
More detailed measures under discussion will include high speed EU extradition warrants so terror suspects could be brought swiftly to trial, and plans to allow law enforcement agencies and governments to track and freeze terrorist funds.
Mr Blair's official spokesman confirmed on Friday that the UK Government would consider introducing new anti-terrorism legislation.