Defense is based on a combination of street and political tactics. In the street, we rode clunkers to reduce the value to the cops of attempts to confiscate or damage bikes. I prefer old road bikes for speed and cheapness.If the cops managed to get on top, we usually made them pay for it later by some political process or another. Finally, we were aggressive if actually attacked, but would go to one lane if just followd and asked "reasonably." If the asking cops disengaged we went back to full width.
There are several ways to defend yourself in the street if things get rough. If cops try to box you in, yuou can scatter in many directions and regroup, or go up a one-way street with flowing traffic. That loses all four-wheeled vehicles and sometimes even motorcycles. At that time any hard-pressed and threatened poeple can escape. When cops are a problem rides should stay near good cover and heavy traffic, avoiding wide-open or empty areas. If a car or motorcycle pulls to the curb to block you, either go around him to the outside or suddenly turn around and SPRINT back the other way-it will take him too long to turn around. Also, most motorcycle cops find the idea of a 20-30mph chase on a wrong-way sidewalk in a yuppie pedestrian environement a little much for them. Also effective on some bike/motorcycle cops is just to ride hard enough through traffic (especially wrong-way!)that the risks to them exceed their appetite for danger. If you have to escape from a trap, take cover and GET INDOORS! Otherwise the cops get ideas about later.
Most importantly of all, the cops need to know that riders will aggressively defend themeslves from arrest. When the notorious Chief Ramsey in DC tried to stop Critical Mass rides with mass arrests, he got clobbered. First try during April 2002 C-M netted only 40 arrests out of 230 riders(intent was to arrest all). We rode curb to curb that day and they couldn't get around us to block us 'till the end. Second try netted 3 out of 40-using a shitload of copcars and a helicopter. We were effective enough in defense that we finally forced the cops to bring out the motorcycles AND bikes of their own, but by that time the Chief was in too much trouble to use this tactic to make mass arrests.
During the war, the cops switched to smashing rear wheels with motorcycles and general intimidation tactics, but had to give up an arrest attempts because the chief was being SUED by the Partnership for Civil Justice over prior illegal mass arrests. After a "fake mass arrest" intimindation assault in which 340 motorcycle cops (at $30K+cost for overtime)attacked a 35 rider C-M, knowing less would be ineffective, threats were made that a blockade team would be standing by to block multiple bridges if the ride failed to check in with an "all-clear" code at the end. The next ride(twice a week during the Blitz) was cop-free! They returned at the next ride in reduced numbers, but harassed people enough that we got a City Council member interested enough to write to the cops and threaten to show up in person. Since then we've had no cop problems, but riding every week burned people out and left us with a controvery about cutting back to the normal once a month and going for fewer, bigger rides.
Also, we found that it wasn't hard to make cops work for nothing. Even if they followed us on 2-wheeled vehicles, they couldn't legally order us to disperse. If we rode in one lane so long as they were present, any action on their part only caused problems for the cops later by getting councilmemebers angry with them. If they left us alone, we had a normal ride, but 14 bikers in two lanes is no bigger an obstacle to cars than 14 bikes and 20 cop motorcycles, with the bikes in a (legal) single lane and the cops in a second lane. Therefore, if we avoided six-lane streets they could not clear even ONE lane of C-M by ANY legal means! It also became obvious that if they chased individual riders at least some of them would force the cops to risk crashing to follow them! Finally, if they ordered a group to pull over, some would comply and some would run(fast and hard). If they chased those who refused to comply, it would pull them off the group and allow the whole group to escape instead. If they ignored the escapees it would sent a powerful message NOT to pull over. Finally, any pullovers for traffic violations would be for tickets that would be hard to enforce on persons with no drivers licenses-and for a $5 ticket anyway. If they used bike registration instead there was no fee to reclaim bikes-and it led to nasty political problems for the cops and some of us threatened to use that as evidence that bike registration was primarily used by the cops to harass people and should be abolished. Since we were getting the ear of the City Council that was not a chance the cops wanted to take any more. Altogether, the results for the cops just was not worth $30K per ride cost to the ruling class.
The final problem is this: the cops may have decided to up the anti enough to drive away messengers, commuters, and other general cyclists and accept that the 'hard core" of anticapitalist riders cannot be stopped or controlled. If they achieve that, they can stay home and ignore the hard core! If this kind of thing happens, you may want to have a couple of "semilegal" rides to keep things safe for the non-hardcore riders, and re-escalate when the cops get tired of following rides that don't do anything.