Two others arrested at the same time have been released.
Danish Justice Minister Lene Espersen said the terrorist case is "the most serious" that the country has known, while Prime Minister Anders Fogh Rasmussen said Denmark must "take the terrorist threat seriously" because of its participation in the Iraq war.
The authorities in Copenhagen provided no details on the target of the attacks.
'A VERY LARGE-SCALE OPERATION': RFE/RL spoke to TIM WILLIAMS, the head of the European Security Program at the London-based Royal United Services Institute, about the alleged terrorist plot uncovered by British security forces today and its implications.
RFE/RL: Does this disclosure show that British security services are doing their job, and are now operating better than they were last year, when they failed to prevent the London transport bombings?
Tim Williams: We are at the very early stages, and there have been a number of arrests. We are currently at 21 people in custody, and the security services and police have clearly done a very, very good job to foil the attacks. But it's also important to remember that today the threat level to the U.K. has been upgraded to the highest level. Now the home secretary has said that is a precautionary measure, but the wording of that threat level is such that we must suspect that perhaps not everyone has been caught yet, and that there's more work to do to ensure that these attacks are foiled.
RFE/RL: Is it inevitable that the United Kingdom will be targeted in view of its policies in Iraq and now Lebanon?
Williams: It is difficult to argue that those things don't contribute, that perhaps the U.K.’s military involvement in Iraq has been a recruiting sergeant for some of these extreme terrorist organizations. But what we most also remember is that these groups have been active for a long time and rather than these policies causing this terrorism, it is something that extreme ideologies and extreme groups exploit for their own ends.
RFE/RL: Using hand luggage as a means of hiding explosive material indicates suicide bombings. Is this confirmation of the radicalization of terrorist activities in Britain?
Williams: The authorities in the U.K. have said that at least a majority of the arrests that have been made have been of British-born people. The information that we have about the attacks so far -- the foiled attacks -- suggests that they were likely to be suicide bombings and so, yes, that is very concerning indeed. But the authorities and security services in the U.K. have warned for a long time that this is a threat.
RFE/RL: Who is behind this plot, which would seem to require considerable resources?
Williams: That is the worrying issue -- the scale of this operation. The press reports are suggesting that there were perhaps up to 10 planned attacks, on 10 separate aircraft. We already have had 21 arrests [and] one could reasonably suspect that there will be more to follow. We are talking about a very large-scale operation and I think that [with] an operation of this size, the immediate suspicion is that there has been foreign involvement in this but all this remains to be seen.
RFE/RL: Have the terrorists been able to achieve something in terms of the widespread disruption of aviation now and in the coming weeks?
Williams: The difficulty to the public and the fact that this will be rolling across all the news television stations, on the radio, and across all the websites will achieve exactly what the terrorists want to achieve: instill fear in the wider population. So, obviously it is a very, very good thing that these attacks have been foiled but the terrorists, through the publicity that they receive, will achieve some of their aims.