In the west, decline seems to have been general: Rome itself was estimated to have fallen to 30,000 inhabitants in the 14th century; Naples, Verona, Milan, Paris and Metz may have fared better but are likely to have been of a similar size around the 8th century, while Spanish cities like Seville, Mérida and Zaragoza appear to have remained important centres and Toledo gained from its promotion to a capital. Other cities were reduced for centuries to just a shadow of their former selves: London seems to have been largely abandoned by the 6th century, but the 7th saw a new settlement just to its west.

Infrastructure suffered widespread decay as a shift from urban living meant that it was less needed - by the 15th century only two of Rome's bridges eight or nine are said to have survived - but many sewers and aqueducts survived centuries of disuse and disrepair, to be restored to operation as medieval population recovered. Administration was largely in the hands of local kings, dukes or counts until the rise of communal government around the 11th century, when growing urban commercial elites began to gain some measure of autonomy, but in a constellation of cities increasingly different to that of the ancient world.

In the East it was a different story, with Constantinople thriving and by some accounts approaching half a million in the 11th century (though I'm a bit sceptical that it was quite so large) and Thessaloniki remaining a major centre. Other cities though appear to have gone into decline as the Empire lost territory its to Arab rivals - Antioch, Ephesus, later Alexandria, to be replaced by the likes of Damascus, Aleppo and later Bursa, Konya and Cairo.

In the case of the eastern half of the empire, the general trend was demographic expansion in the century after the fall of the west. Usually the Limestone Massif in Syria is cited as evidence as there are indications of marginal land being brought under cultivation for the purposes of engaging in intensive commercialized agriculture, but as Gideon Avni has argued in The Byzantine-Islamic Transition in Palestine this also took place in Palestine (note that by the use of the term Palestine I refer to the Roman provinces of Palestina Prima, Palestina Secunda, and Palestina Tertia). That said, the expansion came to a halt in the seventh century. No one reason can be given: war and plague had demographic and economic effects but no one is quite sure just how extensively or how long-term this was. There is clear evidence of climate change, but this began during the period of demographic and economic expansion, so it's also difficult to gauge. For this, see the recent paper of Büntgen et al. in the last issue of Nature Geoscience. Long term trends also tended to be negative towards the future of the Roman city. From the late third or early fourth century, greater impositions began to be imposed on the urban elites while at the same time avenues of escape into the imperial bureaucracy and the clergy opened up. Priorities changed, as did the interest in maintaining the sorts of buildings that a classical city was supposed to have. This was by no means universal, though, and overall this mixture of factors explains why the Roman city largely collapsed in the seventh century in the east. Some have argued for greater continuity (such as Avni, cited above) but I'm not totally convinced - even his data points to major change and dislocation even if the Palestines remained busy and active centers for centuries. The "perfect storm" hypothesis is nicely summarized in D. Stathakopoulos, ‘Crime and Punishment’ in L. Little (ed.), Plague and the End of Antiquity: The Pandemic of 541-750 (Cambridge: Cambridge University Press, 2006): 99-118.

Urbanism inside the lands that remained under Constantinople's sway largely fell apart during these centuries, too. The Frankish pilgrim Arculf describes Ephesos and the Church of St. John as two separate fortified enclaves, and Arab geographers refer to Byzantium as a land of fortresses. This is not to say that all the trappings of urban life disappeared: Amorion had functioning baths and churches, for example, but there is a general picture of demographic decline. Constantinople's main aqueduct was cut in AD 626 and wasn't repaired for a century, suggesting that the city was able to supply itself by a single, smaller aqueduct and rainwater during that time. Some have suggested a population as low as ca. 40,000 during that time, but where these figures are coming from I can't really say. The Parastaseis Syntomoi Chronikai, a curious list of antiquities in Constantinople during the eighth century depicts a rather lifeless and empty city. We'll have to see. The jury is still out on this document. Much like davepx, there is good reason to be skeptical of some of Constantinople's higher population figures even in a period of demographic recovery. The 500,000 figure seems to get tossed around quite a bit, but the only serious study I've seen (by David Jacoby) argues that the city never exceeded 400,000. Unfortunately, without better documentary evidence we're reduced to educated hypotheses.

In the Byzantine lands, most cities kept up large populations. Exceptions were those in the Balkans exposed to "Barbarian" raids. Later on, Arab raids took place in Asia Minor, but quite a few large cities remained there under the theme system.

But in Western Europe, there was a huge trend toward self-sufficiency and ruralization. These went hand-in-hand with reduced trade & reduced population of cities. The Barbarian kings were often at war with one another, and the old Roman population was ruled by these newcomers. The old Roman population became the basis of the European peasants of later centuries.

By the year 600, in most places in Northern Italy/Northern Spain/Gaul/France, the "gentleman" (Knight, or Caballero) was of Germanic origin. He & his men ruled his castle or manor, and just about all goods were locally made, except for some luxuries.

Rome itself, which once had a population of at least a million, by the 800's had about 40,000, and half of them were paupers kept fed and housed by Papal charity.