September 23, 2015
Slovakia on Wednesday announced it was launching a legal challenge.
Wednesday's emergency summit will also focus on tightening EU borders and boosting aid to neighbours of Syria, from where many migrants come.
EU leaders have struggled to find a co-ordinated response to the crisis. On Wednesday the European Commission warned 19 members over breaches to asylum rules.
British Prime Minster David Cameron and French President Francois Hollande held talks on the eve of the meeting, saying that finding a solution to the Syrian conflict would be key to resolving the migrant crisis.
They also agreed more should be done to return migrants who do not have a genuine claim for asylum, a Downing Street spokesman said.
Analysis: Chris Morris, BBC Europe correspondent
European Council President Donald Tusk has told EU leaders that this summit should focus on policies they can implement to help each other, rather than a long list of issues to use to blame each other.
It is both an appeal for practical progress, and an acknowledgement that the political atmosphere has become pretty toxic.
Several leaders will arrive in Brussels angry that their governments have been outvoted on the issue of relocating tens of thousands of refugees. But Mr Tusk will be keen to avoid recrimination.
Instead he wants to focus on improving the security of the EU's external borders, and ensuring that failed asylum seekers get sent home more efficiently.
Most of all though there will be emphasis on increasing funding for UN agencies that deal with refugees in countries bordering Syria, and on closer cooperation with, and more assistance for, Turkey. It is housing more than 2m Syrian refugees, and has become the main gateway into Europe.
In a rare move for an issue involving national sovereignty, EU interior ministers approved the resettlement scheme on Tuesday by majority vote rather than unanimous approval.
EU Migration and Home Affairs Commissioner Dimitris Avramopoulos denied this was harmful to the EU, saying as he arrived for the leaders' summit on Wednesday: "On the contrary, it is a victory for the EU and for all member states."
The deal will see thousands of migrants moved from Italy and Greece to other EU countries. A proposal to take 54,000 migrants from Hungary was dropped.
Finland abstained from the vote. Poland, which had originally opposed the proposal, voted for it.
But its opponents have lashed out. On Wednesday, Slovakia announced a legal challenge.
Prime Minister Robert Fico said: "We will go in two directions: first one, we will file a charge at the [European Court of Justice] in Luxembourg... secondly, we will not implement the [decision] of the interior ministers."
He described the deal as a "diktat" of the majority.
Czech President Milos Zeman said on Tuesday that "only the future will show what a mistake this was".
However, Czech PM Bohuslav Sobotka said on Wednesday it would not launch a court challenge, adding: "Europe must not fall apart on the migration crisis."
Romania said it could manage its allocation but President Klaus Iohannis insisted mandatory quotas were not the answer.
But Luxembourg Foreign Minister Jean Asselborn, who chaired the meeting, said he had "no doubt" opposing countries would implement the measures.
The UN has warned that the relocation alone would not be enough to stabilise the situation.
Close to 480,000 migrants have arrived in Europe by sea this year, and are now reaching European shores at a rate of nearly 6,000 a day.
Who are the 120,000?
- All are migrants "in clear need of international protection" to be resettled from Italy and Greece to other EU member states 15,600 from Italy, 50,400 from Greece in the first year, and a further 54,000 from those countries later dependent on the situation
- Initial screening of asylum applicants carried out in Greece and Italy
- Syrians, Eritreans, Iraqis prioritised
- Financial penalty of 0.002% of GDP for those member countries refusing to accept relocated migrants
- Relocation to accepting countries depends on size of economy and population, average number of asylum applications
- Transfer of individual applicants within two months