Islamabad has lodged a strong protest after the European Parliament refused to meet a hardline Islamist included in a Pakistani Senate delegation.
Mr ul-Haq said he was detained for three hours
The foreign ministry in Islamabad summoned the EU and Belgian ambassadors to deliver the protest.
The EU parliament had said it would not meet an individual who did not meet its "ideals of democracy and equality".
The senator, Maulana Sami ul-Haq, also says he was held for questioning for three hours at Brussels airport.
Mr ul-Haq also heads a seminary in Pakistan that has been called the "University of Jihad".
Pakistan's minister of state for foreign affairs, Khusro Bakhtiyar, told the country's lower house of parliament of the protest.
The house then unanimously passed a resolution condemning the "undiplomatic treatment" afforded the eight-strong Senate delegation by the European Parliament.
Opposition religious party leaders said the EU's attitude showed that Pakistan's foreign policy was a failure.
Mr ul-Haq told the Associated Press he was detained on arrival at Brussels airport on Wednesday.
"I was told by the authorities that I could not go along with other members and I would have to stay for some discussions at the airport.
"They told me that they had received instruction from their interior minister that I should not be allowed in."
Mr ul-Haq said he was allowed to rejoin his party after three hours.
The EU said it would receive the Senate delegation without Mr ul-Haq, but the delegation declined. It is now in Germany.
British Labour MEP, Neena Gill, leader of the EU parliament's South Asia inter-parliamentary committee, said on Wednesday: "While we have members who represent all shades of the political spectrum, we are all working within the framework of a fully functioning democracy.
Graduation at Mr ul-Haq's 'university of jihad' in Pakistan
"We cannot condone therefore individuals who place themselves outside these parameters, for they represent everything we stand against."
Mr ul-Haq is part of the hardline Islamist alliance that forms a strong bloc in Pakistan's parliament.
The BBC's Haroon Rashid visited his seminary in Pakistan's North-West Frontier Province in 2003.
Our correspondent witnessed the graduation of 600 students pledged to fighting a jihad, or holy war, against enemies of their religion.
The convocation was full of slogans in support of Afghanistan's ousted Taleban regime, al-Qaeda's leader Osama Bin Laden and jihad, he reported.
Mr ul-Haq's advice for the Pakistani and Afghan students was to wage holy war until the "evil force" is defeated.